,,minerals ,,in ,,rock ,,sections ,,the ,,practical ,,methods ,,of ,,identifying ,,minerals ,,in ,,rock ,,sections ,,with ,,the ,,microscope ,,especially ,,arranged ,,for ,,students ,,in ,,technical ,,and ,,scientific ,,schools ,,by ,,lea ,mc,i,l,v,a,i,n,e ,,luquer1 ,c4,e41 ,,ph4,d4 ,late ,assistant ,professor of ,mineralogy1 ,columbia ,university1 ,new ,york ,city ,,fourth ,,revised ,,edition ,illustration3 ,logo ,,new ,,york ,d4 ,,van ,,nostrand ,,company1 ,,inc4 #bej ,,fourth ,,avenue ,copyright1 #aiac ,,by ,,lea ,,mcilvaine ,,luquer ,,lancaster ,,press1 ,,inc ,,lancaster1 ,,pa4 ,,preface ,,to ,,fourth ,,revised ,,edition1 #aiac4 ,this edition is practically that of #aijh1 with necessary minor revisions and corrections1 especially the data referring to indices of refraction and strength of double refraction4 ,the values of the refractive indices and are recorded as well as the mean values4 ,references are also given to 8,the ,methods of ,petrographic-,microscopic ,research10 by ,f4 ,e4 ,wright1 #aiab1 for exact 7quantitative7 methods of determining crystal constants4 ,,lea ,,mci4 ,,luquer4 ,,department ,,of ,,mineralogy1 ,,columbia ,,university1 ,,new ,,york1 ,may1 #aiac4 ,,preface ,,to ,,third ,,revised ,,edition1 #aijh4 ,the #aijh edition is essentially the same as that of #aije2 the only changes being in the nature of corrections1 and the addition of a brief description of the ,schrder van der ,kolk method of determining refractive indices and also an abbreviated diagram for determining triclinic feldspars in sections showing both ,carlsbad and ,albite twinning4 ,the ,naumann symbols for crystal forms have also been omitted4 ,,lea ,,mci4 ,,luquer4 ,,department ,,of ,,mineralogy1 ,,columbia ,,university1 ,,new ,,york1 ,april1 #aijh4 ,,preface ,,to ,,revised ,,edition1 #aije4 ,in preparing the revised edition1 ,chapters ,i4 and ,,iv4 have been rewritten and enlarged and the part relating to the determination of the plagioclases has been greatly amplified4 ,many additions have also been made to ,chapter ,,iii4 and the ,becke method1 for the determination of the relative indices of refraction of minerals1 has been given in detail4 ,some new and useful tables have been introduced2 as tables of refractive indices 7mean7 and double refraction 7maximum74 ,a diagram has also been added1 showing the relation existing between strength of double refraction1 interference colors and thickness of section4 ,professor ,e4 ,weinschenk's admirable text-book1 8,die ,gesteinsbildenden ,mineralien01 ,freiburg1 #aija1 has been specially referred to1 and the tables of refractive indices and double refraction have been compiled from ,weinschenk's new ,tables4 ,many new cuts have been added1 among them being semi-ideal drawings1 showing typical outlines of crystal sections1 cleavage1 optical orientation1 etc4 ,in describing the 8,usual ,appearance in ,sections0 of a mineral1 it is of course only possible to mention the usual crystal form in which the mineral occurs in a rock4 ,the crystal may be cut in any way by the plane of the section2 but a general knowledge of the crystal forms will furnish an idea as to the outline1 etc41 that the mineral may show in the section4 ,,lea ,,mci4 ,,luquer4 ,,department ,,of ,,mineralogy1 ,,columbia ,,university1 ,,new ,,york1 ,july1 #aije4 ,,preface ,,to ,,first ,,edition1 #ahih4 ,the identification of minerals in rock sections with the microscope1 including as it does a knowledge of optical mineralogy1 is often difficult for beginners4 ,this may be due to the fact that most of the publications on this subject are quite elaborate in their nature and in either ,french or ,german4 ,while detailed descriptions are very necessary1 and1 in fact1 indispensable for advanced investigation1 they are apt to prove cumbersome and confusing at first4 ,for these reasons this text-book has been prepared by the writer1 with a view of putting before the student only those facts which are absolutely necessary for the proper recognition and identification of the common minerals in rock sections4 ,the footnotes refer the student to standard publications1 in which are given details of the methods and investigations outlined in the text4 ,an elementary knowledge of crystallography and mineralogy is almost indispensable and is here assumed4 ,the microscopic and optical characters of the minerals are recorded in the usual order in which they would be observed with a petrographical microscope4 ,nearly all the rock-forming minerals become transparent in thin sections2 but when opaque1 attention is called to the fact and the characters are recorded as seen with incident light4 ,white light is assumed to be used1 unless otherwise stated4 ,the interference colors recorded in all cases are those given by very thin sections of #j4jc mm4 in thickness4 ,the order followed for the minerals is essentially that of ,rosenbusch 7based on the symmetry of the crystalline form71 with a few exceptions made for convenience1 such as placing pyrrhotite after pyrite and zoisite after epidote4 ,the statements regarding the occurrences of minerals in the common rock-types have been taken mainly from ,les ,minraux des ,roches1 by ,lvy and ,lacroix4 ,the terms axes and directions of elasticity1 used throughout this book1 are very commonly employed in petrographical literature of the present time4 ,these axes and directions should probably more correctly be called axes and directions of vibration or extinction4 ,the reasons for or against the elastic condition of the 8ether0 are of more interest1 however1 to the physicist than to the petrographer4 ,an optical scheme is appended1 with the minerals grouped according to their common optical characters4 ,the writer's thanks are due to ,dr4 ,a4 ,j4 ,moses1 ,professor of ,mineralogy1 and to ,mr4 ,j4 ,f4 ,kemp1 ,professor of ,geology1 for kind suggestions offered during the preparation of this book4 ,,lea ,,mci4 ,,luquer4 ,,department ,,of ,,mineralogy1 ,,columbia ,,university1 ,n4 ,y4 ,,city1 ,october1 #ahih4 ,,table ,,of ,,contents ,,page4 ,,conventions ,,and ,,abbreviations ix ,,chapter ,i4 ,,introductory ,,optics ,,for ,,optical ,,mineralogy4 ,ordinary ,light4--,plane ,polarized ,light4--,effects ,produced by ,crystal ,sections on ,transmitted ,light4--,amorphous ,bodies4--,isotropic ,crystals4--,anisotropic ,crystals4--,double ,refraction4--,uniaxial ,crystals4--,biaxial ,crystals4--,principal ,vibration ,directions4--,axial ,angle4--,dispersion4 #a ,,chapter ,,ii4 ,,petrographical ,,microscope4 ,reflector4--,polarizer4--,nicol ,prism4--,condensing ,lens4--,rotating ,stage4--,objectives4--,analyzer4--,eye- pieces4 #g ,,chapter ,,iii4 ,,investigation ,,of ,,microscopic ,,and ,,optical ,,characters ,,of ,,minerals4 ,opaque ,minerals4 ,transparent ,minerals3 ,with transmitted light3 ,form1 ,color1 ,index of ,refraction and ,relief1 ,becke ,method1 van der ,kolk ,method1 ,cleavage1 ,fracture1 ,inclusions4 ,with polarized transmitted light3 ,pleochroism4 ,with crossed nicols3 ,isotropic ,character1 ,anisotropic ,character1 ,interference ,colors1 ,extinction and ,extinction ,angles1 ,vibration ,directions of ,faster and ,slower ,rays1 ,order of ,interference ,color1 ,strength of ,double ,refraction1 ,determination of ,minerals and ,thickness of ,section1 ,structure4 ,with convergent light3 ,uniaxial ,interference ,figures and ,optical ,character1 ,biaxial ,interference ,figures and ,optical ,character1 ,determination of ,axial ,angle1 ,distinctions between ,orthorhombic1 ,monoclinic and ,triclinic ,sections4 ,resum of ,uses of ,parallel and ,convergent ,polarized ,light4 #ac ,,chapter ,,iv4 ,,microscopic ,,and ,,optical ,,characters ,,of ,,minerals4 ,amorphous ,minerals4--,isometric ,minerals4--,tetragonal ,minerals4--,hexagonal ,minerals4--,orthorhombic ,minerals4--,monoclinic ,minerals4--,triclinic ,minerals4--,mineral ,aggregates4 #ea ,,chapter ,v4 ,,methods ,,of ,,preparing ,,sections4 ,cutting and ,grinding ,machines4--,saws4--,cutting4--,grinding 4 ,plates or ,laps4--,cementing4--,grinding4--,mounti ng4--,cleaning and ,finishing4--,convenient ,apparatus for work4 #aag ,,chapter ,,vi4 ,,chemical ,,and ,,mechanical ,,tests4 ,chemical ,tests on ,crystal in ,section3 ,carbonates1 ,gelatinizing ,silica4--,etched ,figures4--,heating ,section to ,redness4--,methods of ,isolating ,crystals or ,fragments for ,testing3 ,specific ,gravity ,separation1 ,electro-magnetic ,separation1 ,chemical ,separation4--,micro-,chemical ,reactions3 ,borichy's ,method1 ,behren's ,method1 ,special ,tests4 #abi ,,appendix4 ,brief ,scheme of ,classification into ,systems by ,optical ,determinations4--,tables of ,double ,refraction 7maximum7 and ,indices of ,refraction 7mean74--,diagram1 showing relation between strength of double refraction1 interference colors and thickness of section4--,order of ,consolidation of the ,constituent ,minerals in ,plutonic ,rocks4--,optical ,scheme with ,special ,introduction4 #ada ,,index4 #adi ,,conventions ,,and ,,abbreviations4 a ,the assumed direction of the ether vibrations of the fastest ray4 b ,the assumed direction of the ether vibrations of the ray with intermediate velocity4 c ,the assumed direction of the ether vibrations of the slowest ray4 ,in some ,american ,text-,books 7by ,iddings1 ,winchell ,phillips7 a ,x1 b ,y and c ,z4 a ,the assumed direction of the ether vibrations of the faster ray in the given section4#a c ,the assumed direction of the ether vibrations of the slower ray in the given section4 77 ,optical character positive4 77 ,optical character negative4 ,parallel to4 7 7 ,the difference between the indices of refraction of the slowest and fastest rays1 respectively1 transmitted by the crystal1 and indicates in decimals the relative strength of the double refraction4 n ,the mean index of refraction2#b hence 7 7_/#c or 7 #b7_/c4 ,index of refraction of the fastest ray4 ,index of refraction of the slowest ray4 a1 b and relate to the crystallographic axes commonly represented by these letters4 #b;,e the apparent axial angle measured in air1 #b,v being the true angle4 ,bxa ,the acute bisectrix4 ,bxc ,the obtuse bisectrix4 ,ax4 pl4 ,the axial plane1 i4 e41 the plane containing the two 8optic axes40 ,,elongation relates to the appreciable extension often shown by the crystal section4 ,a crystal of long prismatic habit1 cut about parallel to the axis1 would show marked elongation2 while a tabular crystal 7like mica7 would show elongation if cut at right angles to the tabular faces4 ,at times1 of course1 no elongation is appreciable1 as in the case of granular or broken crystals or where the cross-section is essentially square or octagonal4 ,very often the relation of the cleavage is given to the elongation and also to the directions a and c1 which makes it possible to test for a and c even when no marked elongation can be observed4 ,,minerals ,,in ,,rock ,,sections4 ,,chapter ,i4 ,,introductory ,,optics ,,for ,,optical ,,mineralogy4 ,the object of this introduction is merely to give a practical discussion of elementary optics1 as applied to optical mineralogy1#c and no elaborate discussion of this important subject will be attempted4 ,the explanations will be made as simple as possible1 and1 in most cases1 only the optical phenomena will be described without entering into a theoretical discussion as to the cause of these phenomena4 ,light#d may be regarded as transmitted in straight lines by vibrations of the 8ether10 taking place at right angles to the direction of transmission4 ,ordinary light is light with the ether vibrations in all possible directions1 the path described by any particle of ether constantly changing4 ,plane polarized light is simply light with the ether vibrations all parallel to one plane passing through the direction of transmission4 ,by experiment it has been proved that there exists a very close relation between the optical properties of crystals and their other physical properties1 such as form1 color1 transmission of heat1 etc4 ,therefore it is often possible1 by a careful optical investigation of a crystal section1 to determine important crystallographic facts1 even in the absence of any distinct outline4 ,the ,effects ,produced by ,crystals on ,transmitted ,light4 ,consider that a series of optical tests are made on all possible sections#e of crystals in the six systems1 and the manner in which these crystals affect transmitted light ascertained4 ,isotropic ,crystals3 ,it will be found that all sections of ,isometric crystals transmit light with equal velocity in all directions2 that is1 the crystals are optically equivalent in all directions and1 hence1 can produce no double refraction4#f ,in these crystals any section1 however cut1 will transmit all the rays of light1 incident to the surface at right angles1 with no change4#g ,the same is true of ,amorphous bodies1 glass1 etc41 unless they have been subjected to strains or peculiar conditions during cooling4 ,a single image is seen through these isotropic sections4 ,illustration3 ,,fig4 #a4 ,anisotropic ,crystals3 ,it will also be found that nearly all sections 7the exceptions being given later7 of crystals in the remaining five systems1 produce quite a different effect on transmitted light4 ,in these crystals the velocity of transmission of light varies with the vibration direction of the light rays4 ,this property1 called double refraction1#h seems to result from the power of resolving a ray of ordinary light1 with ether vibrations in all directions1 into two rays with ether vibrations in planes at right angles to each other2 the two resulting rays traversing1 usually1 divergent paths in passing through the section4 ,the mineral calcite 7,iceland spar7 exhibits this property to a marked degree1 and in certain sections will show a double image1 ,fig4 #a4 ,that the vibration directions of the two doubly refracted rays are in planes at right angles to each other1 can be easily proved by using a nicol prism4#i ,in most cases the separation of the two images is so slight as not to be perceived by the eye1 and the practical method of testing a crystal section for double refraction will be given later1 p4 #bg4 ,the crystals that show double refraction are further divided into two groups1 uniaxial and biaxial3 7#a7 ,uniaxial1 or those in which the optical characters are symmetrical to one direction1 called an optic axis4 ,this optic axis is the crystallographic vertical axis1 2 and parallel to this direction there is a single value only for the light velocity and no double refraction takes place4#aj ,hence any section parallel to the base 7#jja7 being at right angles to the optic axis1 acts like a section of an isotropic crystal and transmits all the perpendicularly incident rays of light with no change4 ,in any other section double refraction takes place and it can be proved by using a nicol prism that the two rays have ether vibrations1 one in the plane passing through the incident ray and the axis of the crystal1 and the other in a plane at right angles thereto1 hence in the basal plane4 ,this latter ray1 which has a constant velocity1 is called the ordinary ray ,o2 and the other ray1 with velocity varying with the inclination of the section to 1 is called the extraordinary ray ,e4#aa ,the vibration directions are either parallel or symmetrical to cleavage cracks and crystal outlines4 ,in sections parallel to the optic axis1 the two doubly refracted rays have the maximum difference in velocity of transmission1 and hence their vibration directions are called principal vibration directions#ab and the plane containing them an optical principal section4 ,in these sections the refractive index of the ray vibrating parallel to 7extraordinary ray7 is denoted by 1 and that of the ray vibrating parallel to the basal plane 7ordinary ray7 by 4#ac ,to this group belong all ,tetragonal and ,hexagonal crystals4 7#b7 ,biaxial1 or those in which the optical characters are no longer symmetrical to an optic axis but to three planes at right angles to each other 7for monochromatic light74 ,these crystals have1 however 7for light of each wave-length and for each temperature71 two directions parallel to which there is a single value only for the light velocity and hence no double refraction4 ,these directions are called 8optic axes40#ad ,an investigation of these biaxial crystals shows that of all the rays traversing these crystals there are three rays which advance with maximum1 minimum and some intermediate velocity4 ,the vibration directions of these three rays are called the principal vibration directions and are at right angles to each other 7being the intersections of the three planes above referred to74 ,the direction of ether vibration of the fastest ray is denoted by a1 of the slowest ray by c1 and of the ray advancing with intermediate velocity by b4#ae ,each of the three planes1 containing two principal vibration directions1 is called an optical principal section4 ,the index of refraction of the a ray is denoted by 1 of the b ray by 1 and of the c ray by 4 ,to this group belong all crystals in the ,orthorhombic1 ,monoclinic and ,triclinic systems4 ,in the ,orthorhombic system1 the principal vibration directions are parallel to the crystallographic axes2 hence all pinacoidal sections contain two of these principal vibration directions4 ,in all sections parallel to the three crystallographic axes 1 and 1 the vibration directions are parallel or symmetrical to cleavage cracks1 crystal edges1 etc4 ,in the ,monoclinic system1 one principal vibration direction is parallel to the ortho axis 1 the other principal vibration directions are in the plane of symmetry1 at right angles to 1 but are not parallel with either the vertical axis or the clino axis 4 ,in clino pinacoid 7#jaj7 sections the principal vibration directions will make definite angles with crystallographic lines1 such as cleavages or crystal outlines4 ,these angles are called extinction angles4 ,they will vary1 in this system1 with reference to the direction of the axis from a maximum on the clino pinacoid 7#jaj7 to #j on the ortho pinacoid 7#ajj71 when the vibration directions of the two doubly refracted rays will be parallel and at right angles to the plane of symmetry4 ,hence the vibration directions are parallel or symmetrical to cleavages1 edges1 etc41 only in sections parallel to the ortho axis 2 but in all other sections are unsymmetrical4 ,in the ,triclinic system1 the principal vibration directions are not parallel to the crystallographic axes1 and there is no definite relation between these directions and the crystallographic axes2 hence in all possible sections there will be extinction angles4 ,in all biaxial crystals the two optic axes are inclined to each other1 making what is called the axial angle1 #b,v1 the apparent angle measured in air being #b;,e4 ,the optic axes lie in the plane1 called the axial plane1 which contains the principal vibration directions a and c4 ,the axial angles are bisected by these principal vibration directions1 the direction bisecting the acute angle being called the acute bisectrix1 ,bxa1 and that bisecting the obtuse angle the obtuse bisectrix1 ,bxo4 ,an approximate idea of the value of the axial angle can be obtained by the use of the petrographical microscope1 as described later1 p4 #dg4 ,the axial angle is often a convenient distinction between such minerals as muscovite and biotite4 ,the axial angle will vary with the temperature and with light of different wave-length or color1 and this variation is called dispersion of the optic axes4 ,dispersion of the principal vibration directions also takes place in monoclinic and triclinic crystals1 but can be usually disregarded4 ,in closing it is very important to remember that any section of an anisotropic crystal 7not at right angles to an optic axis7 will always transmit two rays of light with different velocities and with vibration directions in planes at right angles to each other4 ,isometric crystals1 of course1 produce no double refraction of light4 ,,chapter ,,ii4 ,,the ,,petrographical ,,microscope4#af ,the petrographical microscope is essentially an ordinary microscope#ag with the following important additional equipment4 ,it must be provided with3 #a a polarizer 7a piece of apparatus for giving polarized light7 placed below the stage2 #b an analyzer 7a piece of apparatus for analyzing the rays of light after they have passed through the polarizer and transparent section7 placed between the objective and the eye2 #c a stage rotating about an axis which is the line of sight of the microscope4 ,a convenient type of microscope is that made by ,seibert of ,wetzlar 7,no4 #aa a71 ,fig4 #b4 ,the reflector a is usually fitted with a plane mirror on one side1 and a parabolic mirror on the other4 ,the plane mirror should be used with sunlight1 and the parabolic mirror with artificial light1 in order to make the rays of light as nearly parallel as possible4 ,the polarizer is in most cases a nicol prism1 set in a suitable frame b1 and made as follows3 ,for the nicol prism 8a cleavage rhombohedron of calcite 7the variety ,iceland spar is universally used in consequence of its transparency7 is obtained1 having four large and two small rhombohedral faces opposite each other4 ,in place of the latter planes1 two new surfaces are cut1 making angles of #fh 7instead of #ga7 with the obtuse vertical edges2 these then form the terminal faces of the prism4 ,in addition to this1 the prism is cut through in the direction ,,hh1 ,fig4 #c1 the parts then polished and cemented together again with ,canada balsam4 ,a ray of light1 ab1 entering the prism1 is divided into two rays polarized at right angles to each other4 ,one of these1 bc1 on meeting the layer of balsam 7whose refractive index is less than that of the ray bc71 suffers total reflection1 and is deflected against the blackened sides of the prism and extinguished4 ,the other1 bd1 passes through and emerges at e1 a completely polarized ray of light1 that is1 a ray with vibrations in one direction only1 and that the direction of the shorter diagonal of the prism40#ah ,the vertical plane through the shorter diagonal may be called the plane of vibration#ai of the nicol4 ,illustration3 ,,fig4 #b4 ,illustration3 ,,fig4 #c4 ,the polarizer must be below the stage c1 and is generally adjusted so as to have its plane of vibration parallel to the ,n4 and ,s4 cross-wire in the eye-piece o4 ,it is important to know the direction of the plane of vibration of the polarizer or lower nicol1 as we can then determine1 when absorption of light occurs in a mineral1 the direction in this mineral parallel to which the absorbed rays are vibrating4 ,the polarizer slides in an outer shell or frame1 and1 by means of a lever d1 can easily be raised or lowered4 ,a convenient test for the location of this plane of vibration of the polarizer is as follows3 ,make use of a section of biotite1 cut at right angles to the basal plane1 hence showing the basal cleavage cracks4 ,biotite has the property of absorbing to a marked extent the light vibrating parallel to these cleavage cracks4 ,rotate such a section on the stage of the microscope until the position of maximum darkness is reached and when such is the case the plane of vibration of the polarizer must be parallel to these cleavage cracks4 ,on the top of the nicol is placed the condensing lens for getting convergent light1 and the adjustments are so arranged that when the nicol is up as far as it will go1 the condensing lens#bj is brought almost in contact with the lower surface of the transparent section resting on the stage4 ,the rotating circular stage e is supported on a suitable frame c1 and arranged so that its axis of rotation coincides with the line of sight of the microscope4 ,the stage is graduated1 and1 by means of an index fixed to the frame1 the angular rotation can always be obtained4 ,it is also provided with two adjusting screws#ba f1 by means of which the axis of rotation can be accurately centered4 ,the method of centering is as follows3 ,bring some prominent mark in the section exactly in coincidence with the intersection of the cross-wires in the eye-piece4 ,rotate the stage #ahj1 and correct one half the error by means of the centering screws1 and the other half by moving the section on the stage4 ,check the result by rotating the stage #ahj again1 and if necessary repeat the corrections in the same way until the adjustment is satisfactory4 ,the objective#bb g screws into the collar h1 which has a slot k1 in the upper portion1 for the introduction of a sensitive color plate1 a one quarter undulation mica plate or a quartz wedge4#bc ,the slot k is so arranged that1 when the sensitive color plates are introduced1 the vibration directions of these plates will make an angle of #de with the planes of vibration of the crossed nicols1 and the interference color will thus be at its maximum intensity4 ,a revolving nose-piece is sometimes used which can be attached to the collar h and arranged to carry two or three objectives1 which can thus be very quickly brought into position for use4 ,this is convenient in passing rapidly from observations with parallel light to observations with convergent light1 which must be made with a high power objective4 ,the difficulty is that recentering is generally required4 ,the modern microscopes are provided with a clip for holding the objectives1 instead of the screw-thread collar h4 ,the analyzer#bd or upper nicol is contained in the frame l1 which is arranged so as to slide in and out of the tube of the microscope4 ,the plane of vibration of the analyzer is fixed by the instrument maker so as to be at right angles to the plane of vibration of the polarizer1 hence in the ,seibert microscope parallel to the ,e4 and ,w4 cross-wire in the eye-piece4 ,consequently when the frame l is pushed into the tube1 the analyzer is introduced in the line of sight between the objective and the observer's eye1 with its plane of vibration at right angles to the plane of vibration of the polarizer2 that is the nicols are crossed4 ,when the nicols are crossed1 if they are properly adjusted1 no light can pass through to the eye and the field of view should be dark4#be ,the eye-piece#bf o fits into the top of the tube1 and1 by means of a little projecting piece fitting into a slot in the frame1 can always be adjusted so as to have its cross-wires parallel to the planes of vibration of the two nicols4#be ,some instruments are provided with an additional slot in the tube between the analyzer and the eye-piece for the introduction of a ,bertrand lens1 which is used to magnify the interference figures produced by convergent light4 ,the first approximate focusing is made by the screw m1 and the fine adjustment by the screw n4 ,in focusing always start with the objective very near the section1 and move it away until the right focus is obtained4 ,never move down towards the section in obtaining the focus1 as there is danger then of striking the objective against the section4 ,for cleaning the microscope xylol can be used1 as it will not injure the lacquer4 ,to lubricate any of the parts use a small quantity of soft tallow1 good clock oil or paraffin oil4 ,,chapter ,,iii4 ,,investigation ,,of ,,microscopic ,,and ,,optical ,,characters ,,of ,,minerals4 ,characters of ,opaque ,minerals1 observed by reflected light4 ,the minerals which remain perfectly opaque in thin rock sections have usually metallic lustre and are very minute in size1 as is the case with the iron ores4 ,when the metallic minerals of a rock specimen are distinctly seen with the unaided eye1 it is rarely necessary to make a section for their determination1 as this can more easily be done by any of the well known blowpipe methods4 ,form1 ,lustre1 ,color1 ,cleavage1 etc41 are recognized in the same way as in the case of macroscopic specimens4 ,in order to make the observations by reflected light alone1 the beam of light from the reflector-mirror must be cut off by holding the hand over the mirror or by moving the mirror4 ,characters of ,transparent ,minerals1 observed by transmitted light4 ,the petrographical microscope for these observations is supposed to be in the condition of an ordinary microscope1 both nicols being out of the field and a strong beam of light coming up through the transparent section from the reflector below4 ,the characters are considered in the order in which they would most naturally appear to the observer during a complete investigation with the microscope4#bg ,illustration3 ,,fig4 #d4--;,idiomorphic augite crystals in camptonite4 ,the section is about at right angles to the vertical axis 1 and shows the intersecting cleavages parallel to the prism of #hg #jf4 ,keene ,valley1 ,n4 ,y4 ,illustration3 ,,fig4 #e4--;,allotriomorphic quartz q1 showing no 8relief10 plagioclase p and decomposed feldspar o in granite4 ,as seen with crossed nicols4 7a7 ,form4#bh ,crystals bounded by planes1 i4 e41 crystals that have formed when conditions were favorable for complete development4 ,such crystals are often called idiomorphic or automorphic1 see ,fig4 #d4 ,crystals that are large in comparison with other accompanying crystals may be called ,phenocrysts4 ,of course crystals1 which are not wholly contained within the rock section1 will only show the outline of the bounding planes cut by this particular section4 ,in some cases1 by a careful study of the outline of several sections of the same mineral and by a measurement of the angles1 it is possible to determine the common 8crystallographic forms0 of the mineral4 ,very misleading outlines may1 however1 be observed2 as for example the triangular outline of a section of a cube1 with the corner truncated4 ,for measuring the angles have the stage accurately centered1 then bring the vertex of the angle to be measured to the intersection of the cross-wires in the eye-piece4 ,bring one of the sides in coincidence with one of the cross-wires4 ,note the reading of the graduated circle and rotate the section until the other side is in coincidence with the same wire4 ,take the reading again and the difference will be the angle required4 ,crystals without bounding planes1 whose surfaces are more or less determined by those of adjacent crystals4 ,such crystals are frequently called allotriomorphic or xenomorphic1#bi see ,fig4 #e4 ,this allotriomorphic form must not be confounded with the worn or rounded boundaries of the component grains in clastic rocks4 ,illustration3 ,,fig4 #f4--;,corroded sanidine crystal in perlite4 ,corroded ,crystals1#cj whose fretted outline is undoubtedly due to corrosive action of the magma1 see ,fig4 #f4 ,broken or ,strained ,crystals4 ,in some cases what were formerly larger individuals have been broken or shattered by dynamic action into much smaller fragments1 see ,fig4 #g1 showing crushed rim of fragments surrounding feldspar 8auge40 ,in other cases an actual bending or distortion of the crystal has taken place1 and again sometimes the effect of mechanical stress is only shown by the so-called 8wavy0 extinction4 ,see p4 #ca and ,fig4 #g1 showing a carlsbad twin of feldspar that has been bent and therefore shows marked 8wavy0 extinction4 ,illustration3 ,,fig4 #g4--,orthoclase 8auge0 7,carlsbad twin71 showing bending and 8wavy0 extinction1 surrounded by crushed rim of mineral fragments4 ,as seen with crossed nicols4 8,augen-gneiss10 ,bedford1 ,n4 ,y4--,b4 #ac4 ,illustration3 ,,fig4 #h4--;,crystallites and ,microlites a4 ,skeleton ,forms b4 ,crystallites1 in general those incipient forms of crystals which have not yet reached a stage of development sufficient to show double refraction1 see ,fig4 #h1 a4 ,quite a number of names are used to describe the different forms that occur4 ,microlites1 more or less completely defined microscopic crystals1 which usually show double refraction1 but cannot be always specifically determined1 see ,fig4 #h1 a4 ,skeleton ,forms1 resulting from rapid cooling of the magma occur in some lavas1 see ,fig4 #h1 b4 7b7 ,color4 ,it must be remembered that the colors observed are always due to transmitted light and may be called 8absorption tints40 ,minerals which in hand specimens are opaque are often colored in sections2 and minerals which are commonly colored may appear colorless in sections4 ,at times color may be given to a section simply by the presence of a great number of minute inclusions4 ,illustration3 ,,fig4 #i4--,olivine crystal in basalt showing 8high relief0 and cleavage4 7c7 ,index of ,refraction4 n sin i_/sin r4 ,this can be approximately determined by the appearance of the surface and outline of a mineral1 that is by its relief4 ,the descriptive terms are of course relative1 but in common practice minerals are said to have medium refraction when the refractive index is between that of balsam 7#a4ed7 and that of calcite 7#a4fj71 strong refraction when higher than #a4fj and weak refraction when lower than #a4ed4 ,in the case of uniaxial and biaxial minerals the mean value of the indices of refraction is used4 ,a mineral which has strong refraction appears to have high relief1 i4 e41 distinct dark contours and a rough or 8shagreened0 surface1#ca which has bright illumination and appears to stand out above the surfaces of the surrounding minerals with weaker refraction1 see ,fig4 #i4 ,a mineral with medium refraction does not show any relief1 hence has a smooth surface and no dark contours1 see ,fig4 #e4 ,a mineral with weak refraction may also appear to have a rough surface1 but not so marked as in the case of a mineral with very strong refraction1 due to the smaller contrast in indices between the mineral and balsam4 ,the practical way of testing for the approximate index of refraction or the relief of a mineral is to make use of the lens for convergent light1 which is placed on top of the lower nicol immediately below the section4 ,by lowering#cb the lens the character of the relief of a mineral section is made very apparent4 ,in order to become familiar with the way in which the 8relief0 or appearance of the surface indicates the strength of the refractive index1 the student may use a long glass slide on which are embedded in balsam 7#a4ed7 small fragments of different minerals1 for example3 ,sodalite 7#a4dhc71 orthoclase 7#a4ebc71 quartz 7#a4edg71 topaz 7#a4fbj71 hornblende 7#a4fca71 augite 7#a4gj71 epidote 7#a4gea71 zircon 7#a4ie7 and rutile 7#b4gab74 ,determinations of refractive indices in sections by the methods of ,chaulnes1#cc ,sorby1#cd or total reflection are accompanied by many difficulties and may fail to give satisfactory results4 ,the ,becke method1 however1 often furnishes a convenient means of determining the relative values of the refractive indices of adjoining minerals or of minerals embedded in balsam4 ,this is of especial service when one of the minerals is known and hence its refractive index4 ,,becke ,,method4#ce ,suppose two adjoining minerals1 in a thin rock section1 to be singly refracting and to have their plane of contact vertical1 i4 e41 parallel to the optic axis of the microscope4 ,let ,a and ,c1 ,fig4 #aj1 be two such sections with the plane of contact ,,oo vertical1 ,a having a lower refractive index than ,c1 and consider only the direction of the rays within the section1 neglecting the refractive effect of the air1 glass and balsam4 ,a beam of transmitted light contains besides normal rays convergent rays1 which pass through the section as indicated in ,fig4 #aj4 ,consider now the cone of light rays ,,gbi4 ,the rays ,o,g on meeting the plane of contact ,,oo will be somewhat concentrated and deflected by the higher refractive index of ,c and will continue as the cone ,,ef4 ,some rays as ,o,h will1 on meeting the contact plane1 be totally reflected and will continue as the cone ,,oe1 while the rest of the rays ,,hi will be dispersed and deflected by the weaker refractive index of ,a1 continuing as the cone ,,od4 ,hence1 more light rays will emerge on the side of the contact plane where the substance of higher refractive index lies1 and there will be a concentration of illumination on this side producing the so-called 8bright line40 ,illustration3 ,,fig4 #aj4 ,in practically making the test with a petrographical microscope remove the polarizer1 analyzer and condensing lens1#cf and introduce below the section a small light-stop1 to reduce in size the cone of incident light4#cg ,the adapter1 holding this light-stop1 should be adjustable so that the stop can be lowered sufficiently to produce the best results4 ,the smaller the contrast between the indices of refraction the smaller the incident cone of light should be1 as best results are obtained when this cone is little larger than twice ,o,h1 ,fig4 #aj1 all the ,o,h rays being then totally reflected4 ,the more of the ,,hi rays that pass through1 the brighter will be the ,,od cone and the less sharp the contrast in illumination4 ,a high power objective#ch should be used1 as those with small aperture and great focal length do not give good results4 ,focus on the dividing plane and adjust until equal illumination is observed on both sides and the trace of the plane resembles a fine thread1 the focal plane being at ,,mn4 ,then raise the objective slightly1 thus moving the focal plane to ,,st1 when a bright line or band will appear on the side of the stronger refracting substance1 the width of the line depending on the contrast between n and n1 becoming narrower as n and n approach each other in value4 ,on raising the objective still further the line broadens and finally disappears4 ,if the objective is lowered instead of raised the reverse phenomenon will take place1 the bright line appearing on the side of the weaker refracting substance4#ci ,when the indices are nearly the same a bright line will appear on both sides and it is important then to make the cone of incident light as small as possible and to select the brighter line4 ,with doubly refracting minerals1 each set of doubly refracted rays would suffer total reflection under somewhat different conditions and there would be no one point ,b 7as in isotropic minerals7 where no bright lines would appear4 ,however1 if the cone of incident light is small enough in diameter1 the test will be practically obtained as described4 ,the ,becke test can often be made with a medium power objective#dj without removing the polarizer and condensing lens1 provided the condensing lens be lowered sufficiently4 ,it may also prove convenient to turn the reflector a little sideways4 ,the thinner the section the more distinct will be the phenomenon4 ,the contact plane must be clear and not coated with opaque decomposition products4 ,when the plane of contact ,,oo deviates considerably from parallelism with the optic axis of the microscope a disturbance of the phenomenon may be expected and no satisfactory results be obtained4 ,in general under these conditions the 8bright line10 both on raising and lowering the objective1 will remain on the side of the overlapping substance1 without regard to the relative values of the indices4 ,the refractive index of the cementing material should not be very much lower than that of either of the thin sections1 as the result would then be to disperse the emerging rays too much and dim the effect4 ,in the case of distinguishing between minerals of high refractive power1 such as augite and garnet1 methyliodide is recommended instead of balsam4 ,in the case of doubly refracting crystals1 the lower nicol 7polarizer7 must be retained1 if it is desired to obtain the relative refractive index of one of the two rays1 either with respect to the balsam or to a ray in an adjoining crystal with a parallel vibration direction4 ,for determining vibration directions of rays and the faster and slower rays in two adjoining sections see later1 pp4 #ca and #cc4 ,the method of ,schrder van der ,kolk#da can also be employed for the same purpose as the ,becke test4 ,the ordinary objective 7,seibert ,no4 ,,ii7 is used1 and the method essentially consists of darkening a portion of the field by means of the finger or a plate inserted below the polarizer4 ,the condenser lens must be lowered beyond the sharp focus of the edge of the shadow 7on raising the condenser the following phenomena are reversed74 ,as the shadow approaches the contact between two minerals a bright line will appear on the edge of the 8far0 mineral1 if its refractive index is the higher1 and a dark line on this same edge if its index is lower than that of the 8near0 mineral4 ,the indices of the two rays in doubly refracting minerals must be determined by the aid of the polarizer4 ,illustration3 ,,fig4 #aa4--;,biotite1 showing perfect cleavage1 in rhyolite4 7d7 ,cleavage1#db which appears as more or less distinct and regular lines or cracks1 see ,figs4 #aa and #ab4 ,these cleavage cracks may be parallel or intersect1 depending on the position of the section relative to the cleavage planes of the crystal4 ,illustration3 ,,fig4 #ab4--;,augite a1 showing good cleavage1 and plagioclase p in diabase4 ,the plagioclase shows 8polysynthetic0 twinning between crossed nicols4 ,illustration3 ,,fig4 #ac4--;,garnet in mica schist1 showing fracture4 ,franconia1 ,n4 ,y4 ,cleavage is sometimes best observed by slightly lowering the condensing lens under the section4 ,when sections show intersecting cleavage cracks it is often possible to recognize the mineral by its known cleavage angle1 as in the case of amphibole and pyroxene4 7e7 ,fracture1 which appears as irregular and non-parallel cracks1 see ,fig4 #ac4 ,illustration3 ,,fig4 #ad4--;,apatite in feldspar a4 ,garnets in quartz1 ,branchville1 ,ct41 b4 ,liquid inclusions of ,,co#b1 some showing gas bubbles1 in quartz c4 ,illustration3 ,,fig4 #ae4--;,enstatite showing 8,schiller0 ,structure4 7f7 ,inclusions1 which may be solid 7either distinct crystals or glass71 fluid or gas1 see ,fig4 #ad4 ,these inclusions are distinguished by the fact that the solid inclusions generally have sharp contours1 the fluid inclusions distinct dark borders1 and the gas inclusions broad dark borders more distinctly marked than those of the fluid inclusions4 ,the fluid inclusions often contain a bubble of gas4 ,the inclusions may have a definite or indefinite position in the crystal in which they occur1 and can sometimes be more distinctly seen by using convergent light or a 8spot0 lens4 7g7 ,schiller ,structure4 8,is that in which cavities of definite form and orientation 7#8negative crystals'7 are developed along certain planes and filled or partially filled by material dissolved out of the enclosing crystal10#dc see ,fig4 #ae4 ,characters ,observed by ,polarized ,light4 ,the polarized light is obtained by passing the beam of light from the reflector through the polarizer or lower nicol1#dd which must be in place below the stage of the microscope4 ,white light is supposed to be used4 ,pleochroism1 that property which all anisotropic minerals have1 to a greater or less extent1 of absorbing certain colored rays in certain directions1 thereby showing different colors in different directions by transmitted light4 ,uniaxial minerals are dichroic showing two differences in color1 produced by the rays which vibrate parallel to the direction of the vertical axis and the plane of the basal axes4 ,biaxial crystals are trichroic showing theoretically three differences of color produced by rays with vibration directions corresponding very nearly to those of the three principal vibration directions4#de ,any given section of a biaxial crystal will1 of course1 appear only dichroic4 ,the practical way of testing for pleochroism is as follows3 ,revolve the stage1 carrying the section1 when a change in the color of the mineral will be noticed1 if it is pleochroic4 ,this pleochroism may appear as an actual change in color or simply as a change in the shade of the same color4 ,at times it may be so weak as hardly to be noticed1 when it is best to make the test with the condensing lens in position immediately under the section1 or by rotating the lower nicol instead of revolving the stage4 ,the color of the light1 vibrating parallel to certain definite crystallographic directions in minerals1 is often very characteristic4 ,in some cases there may be such strong absorption of one of the rays1 that1 when the vibration direction of this ray is over the plane of vibration of the polarizer1 practically no light is transmitted and the section appears dark4 ,strong absorption is characteristic of certain minerals1 such as biotite1 amphibole1 tourmaline and allanite and takes place parallel to definite directions in these minerals4 ,pleochroism may often be noticed with ordinary light in the case of hand specimens4 ,characters ,observed with both ,polarizer and ,analyzer#df in ,position1 that is with 8,crossed ,nicols40 ,when the nicols are accurately crossed1 the field should be quite dark1 and if this is not the case the adjustments must be looked to4 ,the condensing lens should be removed for these tests as they are to be made with parallel light1 but as a matter of convenience instead of removing the condensing lens1 the polarizer with the lens on top may be lowered1 when the results will be about the same as with parallel light4 ,white light is supposed to be used4 ,isotropic ,character4 ,sections of isotropic crystals are perfectly dark and remain so during a complete rotation of the stage through #cfj4 ,the explanation is very simple4 ,light being transmitted by an isotropic crystal in all directions without double refraction2 it follows that the light from the polarizer1 after having passed through the section1 comes to the analyzer still vibrating in the plane of vibration of the polarizer4 ,hence it is entirely cut out by the analyzer4 ,amorphous transparent substances act in the same way and remain dark during complete rotation of the stage4 ,optical anomalies1 i4 e41 double refraction1 may occur in isometric crystals and in amorphous substances that have been subjected to strains4 ,anisotropic ,character4 ,sections of anisotropic crystals1 having the property of double refraction1 produce in general some interference or polarization color1 except as mentioned later4 ,the popular explanation is as follows3 ,illustration3 ,,fig4 #af4 ,in ,fig4 #af1 let ,,pp be the plane of vibration of the polarizer1 and ,a,a the plane of vibration of the analyzer4 ,all the light1 after it has passed through the polarizer1 is vibrating parallel to ,,pp when it reaches the lower side of the transparent crystal1 cdef1 on the stage of the microscope4 ,in this transparent section let ob and oa be the two directions of vibration1 i4 e41 the only two directions parallel to which rays of light can vibrate in passing through the section4 ,let om represent the amplitude of vibration of a ray from the polarizer4 ,when this ray reaches the section it cannot get through it vibrating in the direction om1 but is doubly refracted and of the two resulting rays1 one gets through vibrating in the direction ob and the other vibrating in the direction oa4 ,from m draw perpendiculars to ob and oa4 ,then according to the law of the parallelogram of forces ob will represent the amplitude of vibration of the ray passing through the crystal vibrating in the direction ob1 and oa will represent the amplitude of vibration of the ray passing through the crystal vibrating in the direction oa4 ,we will thus have two rays passing through the crystal1 polarized at right angles to each other4 ,illustration3 ,,fig4 #ag4 ,consider now the general question of the transmission of the doubly refracted rays through a plate4 ,whatever the angle of the parallel incident rays1 each ray as ,,ab1 ,fig4 #ag1 is resolved1 as just described1 into two rays ,,bc and ,,bd1 polarized at right angles to each other and following 7usually7 different paths in the plate4 ,on emergence these follow parallel paths4 ,among the incident rays there are rays ,,eg and ,,fh1 such that one component of ,,fh will emerge at ,d with one of the components of ,,ab1 and one component of ,,ab and one component of ,,eg will emerge at ,c4 ,hence from every point of the upper surface of the plate there will emerge two rays and these rays will have travelled through different paths in the plate with different velocities and will have their vibrations at right angles to each other4 ,when these doubly refracted rays come to the analyzer1 whose plane of vibration is ,a,a1 they cannot get through vibrating in their present directions1 but components of these rays1 such as ot and os1 can get through vibrating parallel to the plane ,a,a4 ,hence we have two series of rays coming to the eye1 polarized in the same plane1 but one set slightly in advance of the other4 ,these rays will 8interfere0 and produce some interference or polarization color4#dg ,in using white light whenever one of two light rays of the same color has suffered a 8retardation0 of just one wave-length 7or even multiple thereof7 the color will be extinguished2 and when the 8retardation0 is one half wave-length 7or even multiple thereof7 the color will be intensified4 ,therefore1 some tints will be extinguished and others intensified1 the combination resulting in the production of some definite interference color4 ,of course in the case of monochromatic light the thickness of the section may be such that 8destructive interference0 takes place producing no color 7darkness74 ,now suppose the stage to be rotated until the section cdef takes the position cdef4 ,the section will be found to be dark and no interference color will be seen4 ,this is due to the fact that the directions of vibration in the section are parallel to the planes of vibration of the crossed nicols1 consequently the light passes through the section still vibrating parallel to the plane ,,pp of the polarizer and is all cut out by the analyzer4 ,darkness will occur every #ij and therefore four times during a complete rotation of the stage4 ,the interference color is also observed to vary in intensity1 but not in color1 and to be at its maximum #de from the positions of darkness4 ,sections of uniaxial crystals at right angles to the optic axis act like isotropic substances and remain dark during a complete rotation of the stage4 ,in the biaxial crystals1 a section at right angles to an 8optic axis0 shows uniform illumination#dh which does not change as the stage is rotated4 ,the only way to find out whether sections that remain dark are isotropic1 or uniaxial at right angles to an optic axis1 is to test them with convergent light as described later1 see p4 #dj4 ,interference ,colors4 ,the interference color shown by any mineral section depends on three factors3 #a the strength of the double refraction 7 the difference between the refractive indices of the slowest and fastest rays72 #b the position of the section in the crystal2 #c the thickness of the crystal section4 ,to eliminate1 so far as possible1 the variation due to the optical orientation of the section1 care must always be taken to obtain the maximum interference color given by the different sections of the same mineral in the rock section4 ,sections giving the maximum color are those parallel to the axis in uniaxial minerals and those parallel to the axial plane in biaxial minerals4#di ,hence such sections in convergent light never show the emergence of an optic axis or a bisectrix2 and furthermore1 other clues1 such as crystal outlines1 cleavage1 pleochroism1 etc41 may help to indicate the favorable section4 ,the influence of the thickness of the section is also important and must be considered4 ,the interference color will rise in the scale 7be higher in order7 as the section becomes thicker4 ,methods of obtaining the thickness of a section are given on pp4 #ce and #cf4 ,when reference is made to definite interference colors the mineral section is supposed to have a thickness of #j4jc mm4 and to be such as to give the maximum color2 and all mineral sections in a rock section are considered to have a uniform thickness4 ,with these precautions in mind the interference colors indicate the strength of the double refraction as follows3#ej ,the colors of minerals with very weak double refraction vary from a bluish-gray to a grayish-white4 ,as the strength of the double refraction increases the colors become the very intense bright tints of the spectrum1 yellow1 red1 blue1 green1 etc41 called the first and second order colors4 ,as the strength of the double refraction still increases the colors pass through the tints of the spectrum in sequence 7called orders71 becoming paler until finally1 when the double refraction is very strong1 the colors become the neutral1 almost colorless tints of the higher orders4 ,the eye must be trained to appreciate the colors of different orders1 and the student is advised to practice with mineral sections1 of known strength of double refraction1 and compare the resulting interference colors with a color chart1#ea or use the interference color diagram at the end of the book4 ,a convenient test can be made with a one fourth undulation mica plate to distinguish between the white of the #a order and the practically white very high order tint4 ,introduce the mica plate in the slot k1 ,fig4 #b1 and the effect will be to produce a marked change in the color of the #a order1 while no change will be observed in the high order color4 ,the exact order of the color can be determined by the use of a quartz wedge1 as described on p4 #ce4 ,abnormal ,interference ,colors#eb may be brought about in several ways4 ,in white light strong absorption of part of the light components may not only alter the color of the mineral but also modify the interference color4 ,the double refraction may be nearly zero for light of a particular color so that the mineral is isotropic for that color1 resulting in a change in the interference color1 as for example the indigo of melilite1 which is nearly isotropic for yellow light4 ,in biaxial minerals the axial angle may be zero for light of a particular color1 resulting also in a modification of the interference color1 as the blue in penninite in sections ,bxa4 ,when the bisectrices are much dispersed there will be no position of darkness between crossed nicols in white light4 ,extinction and ,extinction ,angles4 ,when the section1 see ,fig4 #af1 is in such a position that its directions of vibration are parallel to the planes of vibration of the nicols1 no light can pass through the analyzer1 and the section is dark4 ,hence the light is extinguished and this phenomenon is called extinction4 ,extinction is said to be parallel or symmetrical when the directions of vibration are parallel to any crystallographic lines or directions1 or bisect the angles between these lines4 ,the crystallographic lines or directions may be either cleavages or the similar boundaries of idiomorphic crystals4 ,this kind of extinction is shown by all sections of tetragonal and hexagonal crystals1 by all sections parallel to the crystal axes in orthorhombic crystals and also by the sections parallel to the axis in monoclinic crystals4 ,when the extinction is not symmetrical it is called oblique1 and is shown by all sections of monoclinic crystals 7except those parallel to the axis7 and triclinic crystals4 ,extinction which does not take place over the whole of the section of a single crystal at the same moment1 but passes over the section like a dark wave or shadow1 is said to be 8wavy02 and indicates that the crystal has been subjected to mechanical forces producing a change in the position of the directions of vibration in different parts of the crystal1 see ,fig4 #g4 ,the angle between a direction of vibration in the section and some known crystallographic direction 7as a cleavage or crystal outline7 is called an extinction angle1 and is measured in the following way3 ,find the positions of the directions of vibration in the section1 which must be parallel to the cross-wires when extinction takes place4 ,note the reading on the graduated circle of the stage1 then remove the upper nicol1 in order to get a more distinct view of the field1 and rotate the stage until you bring some known crystallographic line into parallel position with the cross-wire selected as a reference line4 ,take the reading of the graduated circle again1 and the difference between these two readings will be the extinction angle4 ,it can readily be seen that depending on which way the section is rotated1 either a small or a large extinction angle will be obtained1 the two angles being complements of each other4 ,the extinction angle generally recorded is that between the nearest direction of vibration and the vertical axis 4 ,in monoclinic minerals the maximum value of the extinction angle 7the angle of real value in distinguishing the mineral7 can only be obtained from a section parallel to the clino pinacoid 7#jaj72 but in practice sufficiently accurate results can generally be obtained by measuring the extinction angles of all sections of the same mineral1 which seem to be about parallel to the vertical axis 1 and then taking the maximum value obtained4 ,amphibole and pyroxene can easily be distinguished in this way4 ,extinction is generally tested for by revolving the stage1 carrying the section1 until darkness is observed4 ,the principal difficulty1 however1 is caused by the eye failing to appreciate the position of maximum darkness4 ,hence it is better to revolve the stage until the section just becomes dark 7or fades out7 and then revolve further until the section just begins to lighten up again4 ,the mean of the readings1 in these two positions1 will be the one to use4 ,a gypsum test-plate1 so prepared as to give some definite interference color1 as red of the #a order1 may be introduced between the two nicols1 and the section revolved until this color is exactly matched4 ,this perfect matching of color is only possible when the directions of vibration of the section are exactly parallel to the planes of vibration of the nicols1 as otherwise some interference color would be produced by the section and the true color of the test-plate would not appear4 ,the most favorable condition is when the section is colorless and only covers a part of the field of view1 as then the rest of the field shows the color of the test-plate and the exact matching of color is an easy matter4 ,this method of testing can be employed to recognize the very weak double refraction of some minerals1 whose interference colors are such dark grays as not to be noticed without this test4 ,after the test-plate has been introduced1 some slight variation in the color will be observed when the stage carrying the doubly refracting section is rotated4 ,there are other methods#ec for locating these directions of vibration4 ,they depend generally on substituting changes of color for degrees of darkness or on changes in interference figures1 and consist in the use of color test-plates or ,klein's quartz-plate1 or of special oculars prepared by ,bertrand1 ,calderon1 etc4 ,,method ,,of ,,testing ,,for ,,the ,,vibration ,,directions ,,of ,,the ,,faster ,,and ,,slower ,,rays1 ,a ,,and ,c1 ,,in ,a ,,mineral ,,section4 ,a mica or gypsum plate can be used to make this test1 the directions of vibration of the faster and slower rays on these plates being known and marked4#ed ,the crystal section to be tested is placed on the rotating stage1 between crossed nicols1 and turned to the position of extinction1 when these directions of vibration will be parallel to the cross-wires in the eye-piece4 ,the section is then turned #de1 when the interference color will be at its maximum1 and these directions of vibration will make angles of #de with the planes of vibration of the nicols and the cross-wires in the eye-piece4 ,now introduce#ee the test-plate1 between the section and the analyzer1 so that its known directions of vibration also make angles of #de with the cross-wires in the eye-piece4 ,when the test-plate is introduced a new interference color will be noticed which is either higher or lower in the color scale#ef than the original interference color of the mineral section4 ,when the known directions of vibration of the test-plate are superposed over corresponding directions in the mineral section1 the effect is to thicken the section and the interference color rises in the scale4 ,when the directions of vibration of the test-plate are superposed over directions of vibration in the mineral section which are not corresponding1 the effect is to thin the section and the interference color sinks in the scale4 ,for minerals which have very strong double refraction1 as zircons1 so that the interference colors are of the higher orders1 it is advisable to use the method of testing with a quartz wedge4#eg ,if a wedge is inserted between crossed nicols and its c direction inclined at #de to the planes of vibration of the nicols1 then successive interference colors will be seen commencing with the gray of the first order and passing through the colors as shown by a color chart4 ,if now the crystal section lies with its vibration directions also in the diagonal position1 the color of any portion of the quartz wedge will be changed where it covers the section1 the new color being that of a thicker part of the wedge if the c direction in the section lies under the c direction of the wedge4 ,also where the wedge overlaps the section the displacement of the color fringes in the section will be towards the thin edge of the wedge4 ,on the other hand the new color will be that of a thinner part of the wedge when the a direction in the section lies under the c direction of the wedge4 ,in this case the displacement of the color fringes will be towards the thick part of the wedge4 ,this method is very convenient when the crystal section has in any place sloping edges1 showing prismatic color fringes4 ,the movement of these fringes 7as previously described7 is towards the thin edge of the wedge when the vibration directions correspond and away from this edge when they do not correspond4 ,if the wedge when inserted finally 8compensates0 the color of the crystal section 7i4 e41 practically produces an absence of color1 darkness71 then the a in this section must lie under the c in the wedge1 as the effect of the wedge has been to continually thin the section4 ,,determination ,,of ,,order ,,of ,,interference ,,color4 ,this can be determined by use of a quartz wedge or a v4 ,federow mica wedge4#eh ,have the vibration directions of the given section in the diagonal position1 then gradually insert the wedge between the crossed nicols so that the corresponding vibration directions in the section and wedge are crossed1 that is so that the colors are run down until finally dark gray or black is obtained4 ,count the number of times the original interference color reappears1 if n times1 then the color is a red1 blue1 green1 etc41 of n #a order4#ei ,it is often easier to insert the wedge until compensation of the color is obtained2 then as the wedge is pulled out the direct count of the recurrence of the original color will give the order of that color4 ,,method ,,of ,,measuring ,,the ,,strength ,,of ,,the ,,double ,,refraction ,,by ,,von ,,federow ,,mica ,,wedge4#fj 8,the wedge#fa consists of fifteen superposed quarter undulation mica plates1 each about two mm4 shorter than the one beneath it and with their directions of vibration parallel4 ,the series is mounted on a strip of glass and covered with a cover-glass4 ,the wave-length of a middle color may be taken as #efj 7millionths of a millimeter72 hence each quarter undulation mica plate may be considered to possess a phase difference or retardation of #adj 4 ,if1 then1 between the polarizer and analyzer we insert the mica wedge1 so that its direction of vibration of the slower ray1 c1 is at right angles to that of the mineral under examination1 we subtract from the phase difference of the mineral an amount equal to n times #adj 1 in which n represents the number of superposed mica plates in the field4 ,when the mineral appears dark the value evidently corresponds closely to the phase difference of the mineral4#fb ,from the expression e,x1 in which is the phase difference or retardation1 ,x the double refraction1 and e the thickness in millionths of millimeters1 the thickness can be deduced when the double refraction is known or vice versa4 ,if a mineral of known double refraction can be found in the section near the mineral under investigation1#fc the double refraction of the latter can be deduced by measuring the phase difference of the known mineral1 whence e results1 and this substituted in the above formula yields ,x40 ,,method ,,of ,,determining ,,minerals ,,and ,,thickness ,,of ,,section1 ,,by ,,use ,,of ,,table ,,of ,,double ,,refraction 7,,maximum7 ,,and ,,diagram4#fd ,select some easily recognized mineral in the rock section and note the maximum interference color given by any of its sections4#fe ,the strength of the double refraction of this mineral being known1 look up on the diagram the diagonal line corresponding to this double refraction and follow along this line toward the left hand lower corner until the observed interference color is reached1 when the horizontal line will indicate the thickness of the section4 ,the thickness is given in hundredths of millimeters4 ,then in the case of the unknown mineral pick out the section giving the highest interference color and carry along the same horizontal line until this new color is located1#ff then pass up to the right along the diagonal line to the numbers indicating the strength of the double refraction of the unknown mineral4 ,turn to the table where the minerals will be found having about this strength of double refraction4 ,example3 ,in a section of granite 7biotite-granite7 a grain of quartz was selected1 which gave the brightest color4 ,this color was a bright yellowish white and the known double refraction of quartz is #j4jji4 ,following down on this diagonal line until the interference color was reached it was seen that the thickness of the section was about #j4jc mm4 7a very thin section74 ,the section of the undetermined mineral1 giving the highest order color1 showed a bright #b order purple-blue4 ,passing along the #j4jc horizontal line until this color was reached and then up along the diagonal line1 it was seen that the double refraction of this mineral must be about #j4jd4 ,the table gives muscovite and gerite as having about this double refraction4 ,the mineral was proved to be muscovite by its absence of color and relief and by the characteristic cleavage and parallel extinction4 ,illustration3 ,,fig4 #ah4--,sanidine crystal f1 showing ,carlsbad twin 7which1 as it consists of two parts only1 may be called simple71 and quartz q in rhyolite4 ,crossed nicols4 ,structure3 7a7 ,twinning1 generally noticed by the parts of the twin not extinguishing at the same time4 ,it may also be observed1 without crossed nicols1 just as in the case of macroscopic minerals4 ,twinning may be described as3 simple1 ,fig4 #ah2 polysynthetic1 due to repeated twinning after the same law1 ,fig4 #ab2 and crossed or 8gridiron10 due to repeated twinning after two laws1 ,fig4 #ai4 ,illustration3 ,,fig4 #ai4--,microcline1 showing crossed or 8gridiron0 twinning4 ,crossed nicols4 7b7 ,zonal structure1 often only made visible by the zones extinguishing at different times4 ,it may1 however1 be noticed by the zones being of slightly different color1 or by the zonal distribution of inclusions4 ,in the case of the feldspars the zonal structure may be caused either by the crystal being formed of zones of different chemical composition 7the successive zones in the plagioclases growing more acid towards the exterior71 or by ultra-microscopic twinning1#fg ,fig4 #bj4 ,illustration3 ,,fig4 #bj4--,zonal feldspar 7,carlsbad twin7 in trachyte4 ,crossed nicols4 7c7 ,aggregate structure1 being a confused mass of separate little crystals1 scales or grains all extinguishing at different times1 ,fig4 #ba4 ,illustration3 ,,fig4 #ba4--,sphrulites in felsite4 ,ground mass shows aggregate structure4 ,crossed nicols4 7d7 ,sphrulitic structure1 produced by the aggregation1 in a radiate form1 of crystals or crystallites4 ,it is generally easily perceived by the dark cross1 resulting from the extinguishing of the light in those crystals whose directions of vibration are parallel to the planes of vibration of the nicols4 ,when the stage is revolved the arms of the cross do not rotate1 ,fig4 #ba4 ,illustration3 ,,fig4 #bb4--,olivine decomposed to serpentine4 ,the pseudomorphism has been almost complete1 only small portions of the original olivine remaining4 ,the outline of the parent crystal can be quite distinctly seen4 ,crossed nicols4 7e7 ,pseudomorphic structure1 which may be partial or complete and is noticed by the changed portions producing different optical effects from those of the original mineral4 ,sometimes1 although the pseudomorphism has been almost complete1 the form of the original mineral or crystal may still be seen1 ,fig4 #bb4 ,characters ,observed by ,convergent ,polarized ,light4 ,convergent light is obtained by passing the rays of polarized light through a strong condensing lens1 which generally fits like a cap over the top of the polarizer4 ,by means of a suitable adjustment the condensing lens can be brought very close to the lower surface of the section on the stage4 ,the lens thus sends a cone of light through the section1 and used in connection with crossed nicols a series of optical phenomena1 called interference figures1#fh are produced4 ,each direction in which rays are sent is traversed by a minute bundle of parallel rays and these rays extinguish and produce interference colors as already described for parallel light4 ,hence each direction yields a spot or picture in the field of view and from all these spots combined there results an 8interference figure0 or picture1 depending upon the structure of the section for all the directions traversed by the rays4 ,a very high power objective#fi must be used1 and when the eye-piece is removed1 a small image of the interference figure will be seen4 ,in some microscopes an arrangement is made for getting a magnified image of the interference figure1 by retaining the eye-piece and using an additional ,bertrand lens4 ,in order to get good results care must be taken to have strong illumination and the condensing lens close up under the section4 ,the tests are best made with monochromatic light1 but with white light the effects are substantially the same1 the only difference being that the rings and curves are variously colored instead of being simply light and dark4 ,isotropic substances show no interference figures4 ,uniaxial ,interference ,figures4 7a7 ,sections perpendicular to the optic or vertical axis show a dark cross1 with or without colored rings1 ,figs4 #bc and #bd4 ,the figure is symmetrical to the center1 as the optical behavior of uniaxial crystals is symmetrical to the optic axis4 ,illustration3 ,,fig4 #bc4 ,illustration3 ,,fig4 #bd4 ,the arms of the cross are parallel to the planes of vibration of the nicols1 and the figure does not move when the stage carrying the section is rotated4#gj 7b7 ,sections oblique to the optic axis show a portion of a dark cross1 with or without colored rings1 ,fig4 #be4 ,the centre of the cross is not in the axis of rotation1 and as the stage bearing the section is revolved1 the centre of the cross describes a circle1 the arms always maintaining parallel positions4 ,if the section is still more oblique to the optic axis the centre of the interference cross may be outside the field of view1 and only portions of the dark arms will be seen4 ,sections parallel to the optic axis show a vague dark cross1 which1 on rotating the stage1 dissolves into hyperbolic curves 7suggesting biaxial figure71 which very rapidly disappear in the direction of the optic axis4 ,when the interference figure shows colors1 these colors are lower in order in the quadrants containing the optic axis4 ,knowing thus the position of the optic axis the optical character can be obtained by the method for parallel light1 p4 #dc4 ,illustration3 ,,fig4 #be4 ,sections which are thick and have strong double refraction will show the cross and rings clearly and sharply defined1 there being quite a number of rings crowded close together4 ,sections which are very thin and have weak double refraction show only a broad dark cross and no rings4 ,the interference figures will vary between these extremes1 depending on the thickness of the section and the strength of the double refraction4 ,to obtain the most characteristic figures1 observations must be made on sections about perpendicular to the optic axis1 that is sections which remain dark or nearly dark during complete rotation between crossed nicols in parallel light4 ,optical ,character1 ,positive or ,negative4 ,after having obtained an uniaxial interference figure1 test it by means of a undulation mica plate4 ,this plate must be introduced between the objective#ga and the analyzer in such a way that its vibration direction c1 marked on the plate1 makes an angle of #de with the planes of vibration of the nicols4 ,when this is done the interference figure changes1 or may more or less disappear1 two dark spots or blotches being brought prominently into view4 ,if rings are still seen it will be noticed that they have expanded in the quadrants occupied by the dark spots1 and have contracted in the remaining quadrants4 ,this movement of the rings may make it possible to determine the optical character of a section1 which is so oblique to the optic axis that the dark spots are not seen after the introduction of the mica plate4 ,illustration3 ,,fig4 #bf4 ,illustration3 ,,fig4 #bg4 ,if the optical character is positive the line joining these dark spots is perpendicular to the direction c of the mica plate1 see ,fig4 #bf4 ,if negative the line joining the dark spots coincides with the direction c of the mica plate1 see ,fig4 #bg4 ,the 77 and 77 character is easily determined by remembering that the line1 joining the dark spots1 makes the and sign respectively with the direction c of the mica plate4 ,the direction c of the mica plate 7represented in the figures by an arrow7 is of course not seen1 but its position must be borne in mind when making this test4 ,this test can be made with either monochromatic or white light4 ,if the mica plate does not give satisfactory results1 which will be the case when the double refraction of the crystal to be tested is very weak or when the section is very thin1 use a selenite plate1 cut the proper thickness to give the red color of the first order4 ,this plate must be introduced with its vibration direction a 7previously determined7 making an angle of #de with the planes of vibration of the nicols4 ,instead of the dark spots being seen there will appear two blue and two red quadrants4 ,the diagonally opposite quadrants being of the same color4 ,in determining the 77 and 77 character consider the blue quadrants as the equivalent of the dark spots in the preceding case4 ,this test must be made with white light4#gb ,biaxial ,interference ,figures4 ,illustration3 ,,fig4 #bh4 ,illustration3 ,,fig4 #bi4 7a7 ,sections perpendicular to an optic axis exhibit the interference figures shown in ,figs4 #bh and #bi1 the curves being nearly circular and a straight black bar bisecting these curves1 whenever the trace of the plane of the optic axes coincides with the vibration direction of either nicol4 ,as the stage1 carrying the section1 is rotated the bar changes into one arm of a hyperbola and back again into a bar4 ,this arm or bar will rotate in the opposite direction to the motion of the stage4 ,as previously stated sections of biaxial crystals1 perpendicular to an optic axis1 do not remain dark during rotation of the stage between crossed nicols in parallel light4 ,on the contrary these sections remain uniformly illuminated4#gc 7b7 ,sections perpendicular to the acute bisectrix 7see p4 #e71 exhibit interference figures like those shown in ,figs4 #cj and #ca4 ,illustration3 ,,fig4 #cj4 ,illustration3 ,,fig4 #ca4 ,fig4 #cj shows the appearance of the interference figure when the plane of the optic axes is parallel to the plane of vibration of either nicol1 and ,fig4 #ca shows the appearance when this plane is inclined #de to the planes of vibration of the nicols4 ,as the stage1 carrying the section1 is rotated the dark cross seems to dissolve into two branches of a hyperbola1 which again unite to form a cross4 ,in sections perpendicular to a bisectrix1 with a large axial angle1 the figure will appear1 during a rotation of #ij 7in the direction of the hands of a watch71 as in ,fig4 #cb1 top row4 ,when the section is somewhat oblique to an 8optic axis10 the figure appears as in middle row2 and when still more oblique1 as in bottom row4 ,the black centres#gd of the small ellipses and the black hyperbolic curves mark the points of emergence of the optic axes1 and therefore indicate approximately the size of the axial angle1 #b;,e4 ,sections in other positions1 relative to the optic axes1 give interference figures less definite in appearance than those just described2 and the same conditions affect the appearance of all figures as in the case of uniaxial crystals4 ,very thin sections1 of weak double refraction1 may only show indistinct dark crosses or hyperbolic curves1 without any ellipses4 ,the section perpendicular to the acute bisectrix1 which gives the most characteristic interference figure1 cannot generally be recognized except by an examination in convergent light4 ,it is never1 however1 the section giving the maximum interference color4 ,the interference colors in different sections1 normal to the optical elements1 grade downward in the following order3 7#a7 optic normal1 7#b7 obtuse bisectrix1 7#c7 acute bisectrix1 7#d7 optic axis4 ,sometimes cleavage may furnish a clue as to the best section to test as in ,topaz and ,mica1 where the acute bisectrix is normal to the cleavage4 ,illustration3 ,,fig4 #cb4--;,biaxial ,interference ,figures 7from ,reinisch74 ,top row3 ,almost perpendicular to bisectrix1 large axial angle4 ,middle row3 ,somewhat oblique to an 8optic axis40 ,bottom row3 ,more oblique to an 8optic axis40 ,it must be remembered that this uncertainty1 in the choice of sections for testing1 does not exist in uniaxial crystals2 where the best sections are indicated by the fact that they remain dark or nearly so during complete rotation between crossed nicols4 ,the uniaxial or biaxial character of a mineral section1 which only shows an indistinct bar1 may be determined as follows3 ,a bar 7one arm of the cross7 of a uniaxial interference figure moves in the same direction as the rotating stage1 and always remains straight1 while the biaxial bar rotates in the opposite direction to the stage and becomes curved4 ,optical ,character1 ,positive or ,negative4 ,when the axial angle is very small1 so that the interference figure approaches that of a uniaxial crystal1 the methods used for testing uniaxial figures are employed4 ,when1 however1 the axial angle is large1 the following method can be used3 ,after having obtained an interference figure1 from a section as nearly at right angles to the acute bisectrix#ge as possible1 the stage is rotated until the plane of the optic axes 7the trace of which on the plane of the section is the line joining the points of emergence of the two optic axes7 makes an angle of #de with the planes of vibration of the crossed nicols or the cross-wires in the eye-piece4 ,a quartz wedge#gf is now pushed in between the mineral section and the analyzer1#gg so that its axis c 7previously determined and marked on the wedge7 is either at right angles or parallel to the plane of the optic axes of the mineral section4 ,the optical character of the mineral is positive when the ellipses1 surrounding the points of emergence of the two optic axes on the convex sides of the hyperbola1 appear to expand or open out towards the centre when the quartz wedge is pushed in with its axis parallel to the plane of the optic axes4 ,the optical character is negative when the ellipses appear to expand or open out when the wedge is pushed in with its axis at right angles to the plane of the optic axes4 ,as the ellipses expand they move from the points of emergence of the optic axes towards the centre of the interference figure1 and finally open into lemniscates which move outward from the plane of the optic axes4 ,even when the section is very thin and the double refraction very weak1 only the black hyperbolas without ellipses being seen1 the test can be made2 and colored ellipses will appear1 after the pushing in of the quartz wedge1 which will act in the same way as the ellipses of the interference figure4 ,in a section at right angles to the obtuse bisectric these results are all reversed4 ,when the section is perpendicular to one optic axis1 rotate the section until the plane of its optic axes is #de to the planes of vibration of the nicols4 ,the interference figure will now have the appearance as shown in ,fig4 #bi1 the hyperbola being convex towards the acute bisectrix4 ,insert the undulation mica plate1 so that its direction c is parallel to the plane of the optic axes4 ,if the optical character is positive the hyperbola will move towards the acute bisectrix and if negative away from it4 ,when the gypsum plate is used the blue color will appear on the convex side for 77 and on the concave side for 77 minerals4 ,determination of the ,axial ,angle4#gh ,this can be approximately determined with a petrographical microscope1 if equipped with a micrometer eye-piece4 ,have the axial plane of the crystal section in the diagonal position1 ,fig4 #ca2 and measure the distance d from the centre to either hyperbola with a micrometer 7or average the distance to both74 ,then sin ,e d_/,c1 in which ,c is a constant for the same combination of lenses and is obtained by using a crystal section 7mica cleavage7 of known axial angle4 ,for example1 in a mica with #b;,e #ia #ej and d #da4e divisions on the micrometer scale1 ,c d_/sin ,e #eg4gh for that special combination of lenses4 ,the true axial angle can be obtained from the equation sin ,v d_/,c4 ,optical ,distinctions between ,orthorhombic1 ,monoclinic1 and ,triclinic ,crystal ,sections 7perpendicular to acute and obtuse bisectrices74 ,the interference figures are always symmetrical in shape and distribution of color to the planes and axes of symmetry of the crystal system2 hence are most symmetrical in the orthorhombic1 less so in the monoclinic and still less so in the triclinic system4 ,orthorhombic crystals show the figures always in two of the pinacoids and in white light the color distribution will be symmetrical to the trace of the axial plane and the line through the centre at right angles to this trace and also to the central point4 ,monoclinic crystals show the figures in the clino pinacoid or in sections at right angles to this4 ,in white light the color distribution is never symmetrical to two lines1 but is symmetrical either to the trace of the axial plane 7inclined dispersion#gi71 or to the line through the centre at right angles to this trace 7horizontal dispersion71 or to the central point 7crossed dispersion74 ,triclinic crystals show in white light figures with distribution of color unsymmetrical to any line or point4 ,in white light the 8color fringes0 of the hyperbola are due to the 8dispersion0#gi of the optic axes and bisectrices4 ,that is1 for each color 7for light of each wave-length7 there is a particular interference figure2 the overlapping of these superposed figures producing the color fringes4 ,when the axial angle is larger for red light than for violet1 the dispersion is said to be and the interference figure1 in the position of ,fig4 #ca1 will show the hyperbolic curves fringed with red towards the centre 7inside74 ,in general the color with the larger axial angle is nearer the centre of the field4 ,this is due to the extinguishing of light of each color at the axial points1 the resulting colors at these points being produced by white light minus the absorbed color4 ,when the dispersion is the reverse distribution of color fringes will take place4 ,by measuring the axial angle in red and blue light1 this dispersion of the optic axes can also be obtained4 ,,resum ,,of ,,the ,,uses ,,of ,,parallel ,,and ,,convergent ,,polarized ,,light4 ,parallel light is used to detect pleochroism1 to distinguish between isotropic and anisotropic substances1 to study interference colors1 to determine the strength of the double refraction1 to locate directions of vibration1 to measure extinction angles1 to find the directions of vibration of the faster and slower rays1 to determine the relative value of the indices of refraction of the two rays1 and to investigate the crystal structure in general4 ,convergent light is used to distinguish between uniaxial and biaxial crystals1 to determine whether a section that appears to be isotropic is really so or only perpendicular to an optic axis and to determine the optical character1 grade of symmetry 7system71 axial angle and dispersion4 ,,chapter ,,iv4 ,,the ,,microscopic ,,and ,,optical ,,characters ,,of ,,minerals4 ,,opal4 ,,isotropic4 ,,amorphous4 ,,composition3 ,si,o#b4n,h#b,o1 generally soluble in caustic alkalies4 ,usual ,appearance in ,sections3 ,colorless patches1 incrustations or veins1 also at times with sphrulitic structure 7hyalite71 showing interference cross1 of negative character1 between crossed nicols4 ,often shows anomalous double refraction due to strains4 ,the refractive index is very low 7#a4df7 so that the surface of the opal appears rough4 ,,remarks3 ,found as a secondary mineral in many acid volcanic rocks1 rhyolite1 trachyte1 andesite1 etc41 and also in basic basalts4 ,h41 #e4e to #f4e4 ,sp4 gr41 #b4b4 ,,limonite4 ,,amorphous4 ,,composition3 ,fe#b7,,oh7#f1 ,fe#b,o#c1 frequently quite impure4 ,usual ,appearance in ,sections3 ,brownish and opaque1 in very thin sections may be translucent4 ,,remarks3 ,limonite is essentially a decomposition product1 often forming pseudomorphs after ferruginous silicates or halos about the iron ores4 ,,pyrite1 ,pyrites4 ,,isometric4 ,,composition3 ,fe,s#b4 ,usual ,appearance in ,sections3 ,cubes1 pyritohedrons1 combinations of these forms2 or in irregular grains4 ,outline of cross-sections generally square4 ,opaque1 and by reflected light1 bright yellow1 with strong metallic lustre4 ,alters very easily to the oxides of iron 7rust74 ,,remarks3 ,may be present in all kinds of rocks1 igneous1 metamorphic and sedimentary4 ,not noticeably acted on by hydrochloric acid4 ,h41 #f to #f4e4 ,sp4 gr41 #d4i to #e4b4 ,,pyrrhotite1 ,magnetic ,pyrites4 ,,composition3 ,fe,s4 ,distinguished from pyrite by being practically always in irregular masses and not in crystals1 and by bronze yellow color with reflected light4 ,found in basic eruptive rocks1 more rarely in schists4 ,,magnetite1 ,magnetic ,iron ,ore4 ,,isometric4 ,,composition3 ,fe#c,o#d1 often contains ,ti4 ,usual ,appearance in ,sections3 ,grains and crystals 7generally octahedra71 ,fig4 #cc ,b4 ,skeleton crystals frequent in highly ferruginous eruptive rocks4 ,twinning4--,common1 according to ,spinel law4 ,illustration3 ,,fig4 #cc4--;,a1 ,zircon crystals 7isolated from granite7 in balsam1 showing high relief4 ,b1 ,magnetite crystals4 ,c1 ,ilmenite1 showing partial decomposition to leucoxene along crystallographic directions4 ,opaque1 and by reflected light1 bluish-black1 with strong metallic lustre4 ,distinguished from3 ,,hematite1 ,,chromite1 ,,ilmenite and ,,graphite1 by being easily separated from powdered rock by weak magnet4 ,,remarks3 ,very widely distributed in eruptive rocks and crystalline schists4 ,in the eruptive rocks magnetite belongs to the oldest secretions from the magma1 immediately followed by chrysolite1 biotite1 hornblende1 augite1 etc42 hence often appears as inclusions in these and other minerals4 ,magnetite grains may form with other substances pseudomorphs after hornblende1 biotite1 hypersthene1 etc4 ,such pseudomorphs appear to be caused by 8resorption40 ,magnetite is strongly magnetic and soluble in hydrochloric acid4 ,h41 #e4e to #f4e4 ,sp4 gr41 #d4i to #e4b4 ,,chromite4 ,,isometric4 ,,composition3 ,fe,cr#b,o#d4 ,usual ,appearance in ,sections3 ,octahedral crystals1 grains and in the olivine rocks sometimes in dense aggregates4 ,may be surrounded by green1 pleochroic halo of chrome ochre4 ,opaque1 and by reflected light1 brownish-black to black1 with general absence of metallic lustre4 ,usually translucent and brownish on the edges 7by transmitted light71 with a very rough surface due to high index of refraction 7n #b4a74 ,distinguished from3 7a7 ,,magnetite by brownish-black to black color and general absence of metallic lustre 7by reflected light7 and by grains being usually translucent and brownish on the edges 7by transmitted light74 7b7 ,,spinel 7,picotite71 see under ,spinel4 ,,remarks3 ,common in crystalline rocks1 rich in magnesia1 and in serpentine4 ,chromite is not acted on by acids1 is non-magnetic and gives chromium bead test4 ,h41 #e4e4 ,sp4 gr41 #d4c to #e4f4 ,,spinel4 ,,isotropic4 ,,isometric4 ,,composition3 ,mg7,al,o#b7b4 ,pleonaste 7,fe1 ,mg spinel71 ,picotite 7,cr spinel74 ,usual ,appearance in ,sections3 ,octahedral crystals and twins 7after spinel law71 less often in grains4 ,fracture cracks4 ,always optically normal and never decomposed in rocks4 ,usually colorless or dark green 7pleonaste7 to brown 7picotite74 ,the refractive index is high 7n #a4gb1 spinel proper1 to #b4jj1 chrome spinel71 hence the relief is marked and the surface rough4 ,distinguished from3 7a7 ,,garnet when colorless by octahedral shape of crystals 7garnet forms being #aaj and #baa71 when brown 7picotite7 from melanite garnet by common zonal coloration of the latter1 but may require chemical tests4 ,furthermore spinel may have green color and is never decomposed4 7b7 ,,perovskite by the lower index of refraction and the absence of reaction for ,ti4 7c7 ,,chromite chemically or by density or hardness4 ,,remarks3 ,found in gneiss1 granulite1 lherzolite and in regions of contact metamorphism and secondary bedding formations 7picotite71 olivine-basalt and serpentine4 ,spinels are insoluble in hydrochloric acid4 ,h41 #h4 ,sp4 gr41 #c4e to #d4a4 ,,garnet4 ,,isotropic4 ,,isometric4 ,,composition3 ,r#c,r#b7,si,o#d7c4 ,r is ,ca1 ,mg1 ,fe or ,mn2 ,r is ,al1 ,fe1 or ,cr1 rarely ,ti4 ,usual ,appearance in ,sections3 ,irregular grains1 ,fig4 #ac1 or simple crystals1 showing forms 7#aaj7 and 7#baa71 alone or in combination1 ,fig4 #ad b4 ,zonal structure not infrequent1 especially in the titanium varieties1 ,fig4 #cd4 ,color4--,colorless1 or nearly so1 to yellowish1 reddish or brownish4 ,index of ,refraction4--n #a4gej-a4hef1 hence relief high and surface very rough4 ,fracture4--,irregular cracks occur1 but no cleavage noticed4 ,crossed ,nicols3 ,as garnets are isotropic1 sections remain dark during complete rotation4 ,optical anomalies may however occur1 but are generally confined to1 titanium free1 lime garnets and manganese garnets4 ,the effect being to divide the crystal symmetrically into different areas1 8dodecahedral structure40 ,illustration3 ,,fig4 #cd4--;,garnet1 with zonal structure1 in gneiss4 7,from ,cohen47 ,alteration3 ,garnets are usually fresh1 but may be found altered to chlorite or hornblende4 ,distinguished from3 ,,spinel and ,,perovskite4--,see under the latter4 ,,remarks3 ,found principally in granulites1 metamorphic rocks1 contact rocks1 crystalline schists1 etc4 ,certain varieties may be found in eruptive rocks or olivine rocks4 ,may form pegmatitic borders with pyroxene1 spinel1 etc4 ,garnets are practically insoluble in hydrochloric acid4 ,h41 #f4e to #g4e4 ,sp4 gr41 #c4d to #d4c4 ,the insolubility in acids and the high sp4 gr4 help in separating garnet from a powdered rock4 ,,leucite4 ,,isotropic4 ,,isometric4#hj ,,composition3 ,k,al7,si,o#c7b4 ,usual ,appearance in ,sections3 ,crystals or grains1 which vary greatly in size4 ,cross-sections often nearly round4 ,when very small and free from inclusions may be easily overlooked4 ,sometimes the grains are surrounded by tangentially arranged needles of different minerals4 ,illustration3 ,,fig4 #ce4--,leucite1 with radial and tangential inclusions1 ,vesuvius ,lava4 7,from ,cohen47 ,color4--,colorless4 ,index of ,refraction4--n #a4eji1 hence no relief and generally smooth surface4 ,fracture4--,may be noticed1 but no cleavage observed4 ,inclusions4--,common1 radially or zonally arranged1 consisting of minerals or glass1 ,fig4 #ce4 ,crossed ,nicols3 ,the smaller crystals appear isotropic2 the larger crystals show characteristic intersecting systems of twin lamell1 ,fig4 #cf4 ,illustration3 ,,fig4 #cf4--,leucite1 showing complicated1 interpenetration twinning between crossed nicols4 ,double ,refraction4--,very weak 7 #j4jja74 ,in thin sections it may be necessary to use a sensitive color plate to prove double refraction4 ,interference ,colors4--,very low #ast order1 dark gray1 etc4 ,alteration3 ,quite frequent to fibrous or granular zeolites4 ,distinguished from3 ,,analcite--see under analcite4 ,,remarks3 ,almost entirely confined to younger eruptive rocks1 phonolite1 tephrite and other leucite rocks and their tuffs4 ,often found with plagioclase1 nephelite1 augite1 etc4 ,it is more or less attacked by hot hydrochloric acid4 ,h41 #e4e to #f4 ,sp4 gr41 #b4d to #b4e4 ,the isolation of leucite from rock powder can be better accomplished by specific gravity than by chemical methods4 ,,analcite4 ,,isotropic4 ,,isometric4 ,,composition3 ,na,al,si#b,o#f ,h#b,o4 ,usual ,appearance in ,sections3 ,secondary colorless grains1 with no very characteristic microstructure or properties4 ,cleavage parallel to cube 7#ajj7 usually seen4 ,index of refraction low 7n #a4dhh71 hence rather rough surface4 ,between crossed nicols may show optical anomalies1 but not so marked as in garnet4 ,distinguished from3 ,,leucite1 ,,sodalite and ,,nephelite4 ,these minerals are most easily confused with analcite and recourse must be had to chemical tests1 detection of optical anomalies1 gelatinization test or turbidity by heating4 ,,remarks3 ,occurs as a secondary product 7commonly from nephelite or leucite7 in alkali-rich eruptive rocks4 ,considered also as a primary mineral in igneous rocks4#ha ,gelatinizes with hydrochloric acid1 and becomes turbid by heating4 ,h41 #e4e4 ,sp4 gr41 #b4be4 ,,sodalite ,,group4 ,sodalite1 ,haynite 7,hayne7 and ,noselite 7,nosean74 ,,isotropic4 ,,isometric4 ,,composition3 ,sodalite1 #c,na,al,si,o#d ,na,cl4 ,haynite1 #b7,na#b;,ca7,al#b7,si,o#d7b 7,na#b;,ca7,,so#d4 ,noselite1 #b,na#b;,al#b,si#b,o#h ,na#b,,so#d4 ,usual ,appearance in ,sections3 ,dodecahedral rounded crystals or 7,s7 irregular grains4 ,colorless1 yellowish1 brownish1 greenish to deep blue4 ,refractive index low n #a4dhc7,s7 to #a4ejc7;,h71 hence the surface appears rather rough in sodalite and slightly rough in haynite4 ,inclusions1 abundant and characteristic1 often rod-like and arranged regularly1 making section translucent and especially dark at border1 ,fig4 #cg4 ,dodecahedral cleavage sometimes seen4 ,optical anomalies may occur4 ,alteration3 ,takes place easily to aggregates of natrolite1 other zeolites1 mica1 etc4 ,distinguished from3 7a7 ,,one ,,another only by chemical tests4 ,gelatinization test with hydrochloric acid will show in addition to jelly1 salt crystals for 7,s71 abundant gypsum crystals 7,ca,s,o#b #b;,h#b,o7 for 7,h7 and few if any gypsum crystals 7absence of ,ca7 for 7,n74 7,h7 and 7,n7 turn blue when heated1 but test will not work if minerals are decomposed4 ,when treated with hydrochloric acid and nitrate of silver the black sulphide of silver will show on 7,h7 and 7,n71 but the white chloride on 7,s74 7b7 ,,nephelite 7,elolite7 by being isotropic4 7c7 ,,analcite by no turbidity when heated4 ,,remarks3 ,these minerals are found in the basic1 soda-rich rocks4 7,s7 in elolite-syenite also in trachyte and phonolite1 7,h7 and 7,n7 common in phonolite and leucite-porphyry4 ,h41 #e4e to #f4 ,sp4 gr41 #b4c4 ,illustration3 ,,fig4 #cg4--;,haynite1 showing dark centre and border1 in nepelinite4 7,from ,cohen47 ,,perovskite1 ,perofskite4 ,,isotropic4 ,,isometric4 ,,composition3 ,ca,ti,o#c4 ,usual ,appearance in ,sections3 ,microscopic1 octahedral crystals or larger grains1 pale brownish in color and not very transparent1 darker colored in the larger grains4 ,in reflected light grains appear yellowish with adamantine lustre4 ,refractive index very high 7n #b4ch71 hence relief very strong4 ,between crossed nicols1 the little crystals generally appear optically normal and remain dark2 but the larger crystals may show a complicated penetration twinning4 ,distinguished from3 ,,garnet 7melanite7 and ,,spinel 7picotite7 by much higher refractive index and reaction for titanium and by zonal coloration of melanite4 ,when opaque it might be mistaken for the iron ores1 but has no metallic lustre4 ,,remarks3 ,found in the younger basic eruptive rocks1 especially melilite-basalt4 ,commonly associated with the iron ores nephelite1 augite and chrysolite4 ,insoluble in hydrochloric acid4 ,h41 #e4e4 ,sp4 gr41 #d4a4 ,,rutile4 ,,anisotropic4 ,,uniaxial4 ,,tetragonal4 ,,composition3 ,ti,o#b4 c ,,elongation 4 ,usual ,appearance in ,sections3 ,sharp1 elongated1 prismatic crystals when microscopic1 but granular when the individuals are large4 ,grains may be almost opaque1 with adamantine lustre by reflected light4 ,knee- or heart-shaped twins1 ,figs4 #ch and #ci1 common in the smaller crystals1 the larger individuals may also show geniculated twinning4 ,small crystals sometimes form net-shaped groups 7sagenite71 by crossing one another at angles of #fj4 ,pleochroic halos may surround crystals4 ,color1 yellowish to reddish-brown4 ,index of refraction very high 7n #b4gab1 #b4faf1 #b4ijc71 hence relief marked and surface very rough4 ,prismatic cleavage present in larger individuals1 not observed in microscopic crystals4 ,pleochroism and strong absorption may be noticed1 especially in the larger grains1 but may fail entirely4 ,crossed ,nicols3 ,double refraction very strong 7 #j4bhg74 ,interference colors#hb very high order1 only seen in the microlitic crystals which do not appear dark due to total reflection2 in other cases may not show at all4 ,extinction parallel to prisms4 ,in convergent light optical character 774 ,alteration3 ,may take place to a white or yellowish1 fibrous or granular substance1 strongly refracting1 and similar to the alteration product of ilmenite4 ,may be surrounded by grains of titanite4 ,distinguished from3 ,the ,,opaque ,,ores by adamantine lustre with reflected light2 ,,zircon and ,,cassiterite in concentrates by chemical tests4 ,may not be possible to distinguish from cassiterite in sections4 ,,remarks3 ,found in the metamorphic schists1 amphibolites1 slates1 contact and fragmentary rocks1 etc42 also as inclusions in quartz and mica4 ,especially common as a secondary product of titaniferous hornblende and biotite4 ,the 8sagenite0 webs of the decomposed micas are probably secondary4 ,rutile is insoluble in hydrochloric acid4 ,h41 #f to #f4e4 ,sp4 gr41 #d4b4 ,it is easily separated from rock powder by its insolubility in acid and its high sp4 gr4 ,illustration3 ,rutile twins4 ,,fig4 #ch4--,twin plane 7#aja74 ,,fig4 #ci4--,twin plane 7#cja74 ,,zircon4 ,,anisotropic4 ,,uniaxial4 ,,tetragonal4 ,,composition3 ,zr,si,o#d4 c 4 ,,elongation c4 ,usual ,appearance in ,sections3 ,small1 short prismatic crystals1 ,fig4 #cc;,a1 and grains4 ,shell-like 7zonal7 structure may be noticed4 ,when enclosed in black-mica1 hornblende1 cordierite1 etc41 often surrounded by characteristic pleochroic halos4#hc ,color4--,colorless1 rarely pale brownish4 ,index of ,refraction4--n #a4ie1 7 #a4ica1 #a4iic7 hence relief very high and surface rough4 ,polarized ,light3 ,pleochroism4--,not usually noticeable4 ,crossed ,nicols3 ,double ,refraction4--,very strong 7 #j4jfb74 ,interference ,colors4--,very high 7#dth7 order1 minute crystals show brilliant colors4 ,extinction4--,as zircon is uniaxial1 basal sections remain dark during rotation of stage4 ,in all other sections extinction is parallel to 4 ,convergent ,light3 ,basal sections1 which are large enough to give interference figures1 show several rings in addition to dark cross4 ,optical character 774 ,alteration3 ,very rarely takes place4 ,distinguished from3 7a7 ,,apatite4--,by much higher relief and stronger double refraction4 7b7 ,,titanite4--,by uniaxial character4 7c7 ,,rutile4--,see under the latter4 ,easily confused with xenotime1 which1 however1 has higher interference colors and more distinct pleochroism2 but chemical tests may be necessary4 ,,remarks3 ,found widely distributed but not in quantity in eruptive and metamorphic rocks4 ,occurs in granite1 syenite1 diorite1 gabbro1 gneiss1 etc4 ,it is one of the oldest constituents of the rocks in which it occurs1 and may often be found as inclusions in the ferro-magnesium minerals4 ,zircon is insoluble in hydrochloric acid4 ,h41 #g4e4 ,sp4 gr41 #d4e to #d4g4 ,it can easily be separated from rock powder on account of its high sp4 gr41 insolubility in acid and non-magnetic properties4 ,the crystals can then be examined separately1 or chemical tests made to prove the presence of ,zr4 ,,scapolite ,,group1 ,wernerite1 etc4 ,,anisotropic4 ,,uniaxial4 ,,tetragonal4 ,,composition3 ,silicates of ,ca1 ,al a4 ,,elongation a4 and ,na4 ,usual ,appearance in ,sections3 ,colorless grains1 lath-like individuals or prisms 7dipyre1 in contact metamorphic limestone74 ,index of refraction 7n #a4eea to #a4ehd21 #a4edb to #a4eeh1 #a4eee to #a4eig7 about the same as quartz1 hence usually no relief and surface smooth4 ,cleavage distinct parallel to square prism4 ,inclusions 7carbonaceous7 may be abundant in contact rocks4 ,crossed ,nicols3 ,double refraction usually strong1 but varies 7 #j4jac to #j4jci71 increases with the ,ca percentage4 ,interference colors upper #ast or #b;d order1 more brilliant than those of most of the colorless minerals4 ,basal sections 7showing cleavages intersecting at #ij7 isotropic4 ,extinction parallel in longitudinal sections4 ,in convergent light basal sections show distinct uniaxial interference figure2 optical character 774 ,alteration3 ,takes place easily to a fibrous substance or to kaolin1 muscovite1 etc4 ,distinguished from3 7a7 ,,feldspars 7not showing twinning7 and ,,iolite 7,cordierite7 by uniaxial character1 cleavage and higher order interference colors4 7b7 ,,quartz by cleavage1 higher order interference colors and optical character2 quartz is 77 7c7 ,,apatite 7in grains7 by lower index of refraction1 cleavage and higher order interference colors4 ,,remarks3 ,found especially in metamorphosed diabases and gabbros 7,norwegian72 also in gneisses1 crystalline schists1 metamorphosed limestones1 etc4 ,dipyre occurs in contact zones of limestones and schists1 where it might be confused with andalusite1 but cross-sections show uniaxial character4 ,the minerals of this group are more or less soluble in hydrochloric acid4 ,when the scapolite contains ,cl1 the following test can be made on fresh material4 ,treat with a solution of silver nitrate in hydrofluoric acid and the jelly will be impregnated with chloride of silver which will turn brown4 ,h41 #e4e4 ,sp4 gr41 #b4fh4 ,,vesuvianite1 ,idocrase4 ,,anisotropic4 ,,uniaxial4 ,,tetragonal4 ,,composition3 a4 ,,elongation a4 ,ca#f;,al#c7,,oh4,f77,si,o#d7f4 ,usual ,appearance in ,sections3 ,grains or prismatic crystals4 ,almost colorless to reddish 7when containing ,mn74 ,index of refraction high 7n #a4gae1 #a4gja to #a4gbf1 #a4gje to #a4gcb71 hence relief marked and surface rough4 ,cleavage imperfect1 parallel to prism4 ,pleochroism generally very faint4 ,crossed ,nicols3 ,double refraction very weak 7 #j4jja to #j4jjf71 may vary in different portions of the same crystal 7optical anomalies due to the mineral being at times a mixture of isomorphous individuals74 ,interference colors very low #ast order1 dark gray1 etc41 may often appear zonal4 ,basal sections isotropic when normal1 but may show division into biaxial portions4 ,extinction parallel in sections elongated axis4 ,in convergent light basal sections show a faint cross when normal2 optical character generally 774 ,alteration3 ,not known in rock-making vesuvianite4 ,distinguished from3 ,,epidote 7,pistacite7 by the very low order interference colors4 ,,corundum by weaker double refraction4 ,,garnet 7,grossularite71 ,,zoisite1 and ,,apatite may be easily confused with this mineral and hard to distinguish from it4 ,,remarks3 ,found in limestones1 that have undergone alteration by contact with igneous rocks1 and in metamorphic schists4 ,also may occur in dense 7nephrite-like7 aggregates in serpentine4 ,it is insoluble in hydrochloric acid4 ,h41 #f4e4 ,sp4 gr41 #c4c to #c4h4 ,,melilite4 ,,anisotropic4 ,,uniaxial4 ,,tetragonal4 ,,composition3 ,na#b7;,ca1,mg7#aa7;,al1,fe7#d,si#i,o#cf 84 a4 ,,elongation c4 ,usual ,appearance in ,sections3 ,almost colorless1 tabular 7 base7 crystals or irregular grains or shreds4 ,sections very commonly lath-shaped1 and often characterized by the peculiar 8peg-structure10#hd the lines or markings being 7 elongation of the section74 ,index of refraction 7n #a4fcj1 #a4fbi1 #a4fca7 higher than that of the other associated colorless materials1 hence relief rather marked4 ,cleavage1 parallel to base1 very imperfect4 ,crossed ,nicols3 ,double refraction very weak 7 #j4jjc71 and diminishes with a decrease of ,al4 ,interference colors the lower #ast order1 grays1 etc42 anomalous interference colors may show4 ,extinction parallel to cleavage or the peculiar markings or lines4 ,optical character usually 771 but when poor in ,al 774 ,alteration3 ,takes place frequently to a fibrous aggregate4 ,distinguished from3 ,,nephelite and ,,feldspar by higher relief1 shape1 8peg-structure0 and usual dull appearance with reflected light4 ,,remarks3 ,abundant in the leucite and nephelite rocks 7associated with these minerals and with augite1 perovskite and chrysolite71 and takes the place of a feldspar in the melilite-basalt4 ,it gelatinizes easily with hydrochloric acid4 ,h41 #e4 ,sp4 gr41 #b4i4 ,,graphite4 ,,hexagonal4 ,,composition3 ,c4 ,usual ,appearance in ,sections3 ,minute particles1 or flakes and grains of irregular shape1 seldom crystallized4 ,opaque1 and by reflected light1 black with metallic lustre4 ,distinguished from3 ,the similarly appearing ores by its insolubility in acids and the possibility of making it disappear by heating4 ,,remarks3 ,graphite is widely distributed in the oldest rock formations1 especially in the schists4 ,it is often associated with rutile and the iron oxides4 ,graphite is not acted on by acids4 ,h41 #a to #b4 ,sp4 gr41 #b4ji to #b4be4 ,it is burnt with great difficulty in thin sections on platinum foil2 but this test may vary1 in many cases the graphite 7when in bladed flakes7 not being consumed even after long heating4 ,when heated it may expand into worm-like forms4 ,carbonaceous ,matter4--,occurs in opaque1 grayish-black particles having no lustre2 and is found finely disseminated1 sometimes in larger aggregations1 in clay slates1 limestones1 etc4 ,,hematite4 ,,hexagonal4 ,,composition3 ,fe#b,o#c4 ,usual ,appearance in ,sections3 ,irregular scales1 minute grains or earthy4 ,distinct crystalline forms not often observed in rocks4 ,opaque1 and by reflected light1 black with metallic lustre1 or red without lustre4 ,may also be transparent in red tints4 ,no marked pleochroism observed4 ,,remarks3 ,found widely distributed in acid eruptive rocks1 crystalline schists1 etc4 ,also as inclusions in minerals1 and as a red pigment in many rocks4 ,it is insoluble in hydrochloric acid1 and non-magnetic1 unless attached to grains of magnetite4 ,h41 #e4e to #f4e4 ,sp4 gr41 #d4i to #e4c4 ,,ilmenite1 ,menaccanite4 ,,hexagonal4 ,,composition3 7,fe,ti7#b,o#c4 ,usual ,appearance in ,sections3 ,irregular masses1 without crystallographic outline1 rhombohedral crystals1 or skeleton-like growths4 ,also in brownish1 translucent mica-like forms4 ,opaque1 and by reflected light1 iron-black with metallic lustre4 ,when translucent3 pleochroism brown to yellow2 double refraction not very strong2 optically 774 ,alteration3 ,often takes place to a whitish1 strongly refracting1 substance only slightly transparent1 called leucoxene4 ,this alteration product frequently develops along definite rhombohedral directions1 ,fig4 #cc ,c4 ,also a change to titanite or rutile may occur1 or the ilmenite may be surrounded by these minerals4 ,distinguished from3 ,,magnetite and ,,hematite4--,by whitish1 strongly refracting decomposition product4 ,at times the distinction may be very difficult4 ,,remarks3 ,ilmenite occurs principally in the soda-rich and basic eruptive rocks4 ,the mica-like form is limited to the porphyritic eruptives4 ,the brown pigment in the plagioclase of certain gabbros may be ilmenite4 ,it is attacked slowly by hot hydrochloric acid1 and the solution when heated with tin becomes violet4 ,pure ilmenite is indifferent towards the magnet1 hence strong magnetic properties would indicate a mixture with magnetite4 ,h41 #e to #f4 ,sp4 gr41 #d4e to #e4 ,,corundum4 ,,anisotropic4 ,,uniaxial4 ,,hexagonal4 ,,composition3 ,al#b,o#c4 c a4 ,usual ,appearance in ,sections3 ,pyramidal or prismatic crystals1 grains or basal plates4 ,zonal structure or twinning may be noticed4 ,colorless or with patches of blue4 ,index of refraction high 7n #a4gff1 #a4gfj1 #a4gfi71 hence relief well marked and surface very rough4 ,rhombohedral cleavage may show in larger individuals4 ,pleochroism only marked when color is deep4 ,crossed ,nicols3 ,double refraction weak 7 #j4jji71 like quartz4 ,interference colors middle #ast order1 white to yellow4 ,extinction parallel in elongated sections4 ,optical anomalies very rarely noticed in microscopic individuals4 ,in convergent light basal sections show a rather indistinct cross2 optical character 774 ,distinguished from3 7a7 ,,apatite and ,,vesuvianite by brighter interference colors4 7b7 ,,tourmaline 7light colored7 by not having such strong absorption4 7c7 ,,cyanite by uniaxial character4 ,corundum may need to be isolated from the rock in order to be determined with certainty4 ,,remarks3 found in contact metamorphic rocks1 eruptive rocks1 granular limestones1 etc4 ,it is insoluble in hydrochloric acid4 ,when rock sections are ground with emery1 care must be taken not to confuse grains of emery with corundum in the rock4 ,h41 #i4 ,sp4 gr41 #c4i to #d4 ,,quartz4 ,,anisotropic4 ,,uniaxial4 ,,hexagonal4 ,,composition3 ,si,o#b4 c4 ,usual ,appearance in ,sections3 ,allotriomorphic in the granitoid rocks1 when apparently the last mineral to form1 ,fig4 #e4 ,more or less chemically corroded pyramidal crystals 7with cross-sections six-sided or rhombic with an angle of about #ajj7 in the porphyritic rocks4 ,rounded or angular grains in the 8clastic0 rocks2 granular mosaic in crystalline schists and contact rocks1 very rarely in distinct crystals in any rocks4 ,may at times be mutually interpenetrated with an acid feldspar 7the areas of quartz and feldspar extinguishing as entire crystals71 producing 8micro-pegmatitic0 structure1 ,fig4 #fg4 ,finally may appear as pseudomorphs after other minerals1 but may then consist of some of the other forms of silica4 ,color4--,colorless1 although by reflected light it may appear colored or cloudy if it contain many inclusions4 ,index of ,refraction4--n #a4edg 7 #a4edd1 #a4eec7 hence no relief and surface smooth4 ,cleavage44--,rarely noticed1 an important fact in determining quartz4 ,quartz breaks irregularly4 ,inclusions4--,minute fluid1 gas and mineral inclusions1 often in irregular trains1 are very characteristic of quartz in granite rocks and crystalline schists4 ,the inclusions are not so abundant in porphyritic rocks1 but a few glass inclusions may occur1 filling up 8negative0 crystals in the quartz4 ,rutile1 amphibole1 etc41 may occur as needle-like inclusions in quartz4 ,polarized ,light3 ,pleochroism4--,none4 ,crossed ,nicols3 ,double ,refraction4--,weak 7 #j4jji74 ,interference ,colors4--,the middle #ast order1 white1 yellow1 etc4 ,extinction4--,as quartz is uniaxial1 basal sections remain dark during a complete rotation of stage4 ,in the other sections extinction is not characteristic1 due to the absence of cleavage and crystallographic outlines4 ,thin sections do not show circular polarization4 ,convergent ,light3 ,basal sections show a dark cross1 without any rings4 ,optical character 774 ,alteration3 ,does not take place1 so quartz always appears fresh and unweathered in sections4 ,distinguished from3 7a7 ,,sanidine 7in fresh grains74--,by use of convergent light4 ,feldspar is biaxial1 or sections which appear uniaxial are 774 7b7 ,,nephelite4--,by almost entire absence of hexagonal outline1 stronger double refraction1 fresh1 unweathered appearance and 77 optical character4 7c7 ,,iolite 7,cordierite71 ,,scapolite and ,,topaz4--,see under the latter minerals4 ,quartz may be distinguished from all silicates by being dissolved without residue in hydrofluoric acid4 ,,remarks3 ,quartz occurs widely distributed1 as in the great sandstone formations4 ,it is also a characteristic mineral of all acidic rocks1 being common in granite1 aplite1 rhyolite1 quartz-porphyry1 quartz-diorite1 dacite1 etc4 ,quartz is very brittle and hence is a good indicator of the dynamic forces which have affected the rocks4 ,it may show traces of mechanical deformation by peripheral shattering of the larger grains or by 8wavy extinction02#he and also evidences of chemical corrosion by curved and looped contours4 ,in some diabases the quartz may be surrounded by a rim of hornblende or augite needles 78quartz augen074 8,cataclastic0 quartz may be biaxial4 ,the 8secondary enlargement0 of quartz in clastic rocks may be noticed by the deposition of silica in crystallographic orientation around the clastic grains1#hf the new portion extinguishing at the same time as the core4 ,quartz is not attacked by ordinary acids4 ,h41 #g4 ,sp4 gr41 #b4f to #b4g4 ,chalcedony4--,this variety of ,si,o#b has a radially fibrous structure and shelly parting4 ,it may form sphrulites1 central sections through which show a dark cross between crossed nicols1 or line cavities in rocks4 ,the index of refraction is a little lower than for ordinary quartz4 ,the optical character is 771 which must be determined by a mica or gypsum plate1 a4 ,elongation a4 ,chalcedony occurs in the ground mass of very silicious porphyritic rocks1 which have microfelsitic development2 and is found as a secondary mineral in all kinds of silicate rocks4 ,,tridymite4 ,usual ,appearance in ,sections3 ,this form of ,si,o#b1 which is soluble in boiling caustic soda1 appears in 8tile-like0 aggregates of minute colorless plates 7pseudo-hexagonal7 and is always secondary4 ,the refractive index is extremely low 7n #a4dgg71 hence the surface appears rough4 ,between crossed nicols the interference colors are very low in order 7- #j4jjb71 and the tablets may show a division into different areas 7optical anomalies74 ,in convergent light an indistinct biaxial figure is generally seen4 ,,remarks3 ,chiefly a volcanic mineral1 found in rhyolite1 trachyte and andesite4 ,commonly associated with opal and chalcedony4 ,,calcite4 ,,anisotropic4 ,,uniaxial4 ,,hexagonal4 ,,composition3 ,ca,c,o#c4 ,ca may be replaced by small quantities of ,mg1 ,fe1 ,mn1 etc4 a4 ,usual ,appearance in ,sections3 ,grains and aggregates4 ,may be fibrous or olitic4 ,only in crystals in certain rocks4#hg ,illustration3 ,,fig4 #dj4--;,calcite1 crossed twin lamell1 in granular limestone4 7,from ,cohen47 ,twinning4--,polysynthetic1 parallel to one or more faces of ,r4 7#ajab74 ,very common in crystalline limestones1 and may have been produced by pressure or by the grinding of the section4 ,shows itself between crossed nicols as a series of light and dark bands1 parallel or intersecting1 ,fig4 #dj1 about parallel to longer diagonal of cleavage rhombs4 ,when the composition face of the twins is oblique to the face of the section1 interference colors can be seen without the analyzer4 ,color4--,colorless when pure1 but may appear colored by transmitted light1 due to organic pigments4 ,index of ,refraction4--n #a4fja 7 #a4dhg1 #a4fei7 hence with ordinary light relief not marked4 ,due to the great variation in refractive indices of the two rays1 with polarized light1 the surface will appear either quite smooth or rather rough1 depending upon which vibration direction lies over the plane of the polarizer4 ,this marked variation in appearance 7sometimes called 8twinkling07 serves as a good test for calcite4 ,illustration3 ,,fig4 #da4--;,calcite1 section parallel to face of rhombohedron1 showing rhombohedral cleavage4 7,from ,cohen47 ,cleavage4--,parallel to unit rhombohedron 7#ajaa71 appearing in thin sections as many sharp cracks1 whose angles of intersection depend on the position of the section1 ,fig4 #da4 ,newton's colors may be seen along cleavage cracks4 ,polarized ,light3 ,pleochroism4--,none4 ,crossed ,nicols3 ,double ,refraction4--,very strong 7 #j4agb74 ,interference ,colors4--,pale1 iridescent colors of very high order4 ,extinction4--,as calcite is uniaxial1 basal sections remain dark during rotation4 ,extinction angles with respect to the cleavage cracks vary with the position of the section4 ,convergent ,light3 ,basal sections1 even when very thin1 show distinct interference figure1 with cross and rings4 ,optical character 774 ,distinguished from3 7a7 ,other ,,carbonates4--,by ease with which it is attacked by cold dilute acids1 test can be made on slide after removing cover4 7b7 ,,magnesium-,,bearing ,,calcite4--,by micro-chemical tests4 7c7 ,,titanite 7,sphene74--,see under the latter4 ,,remarks3 ,calcite is very widely distributed1 in addition to the extensive sedimentary limestone deposits4 ,common limestone consists of dense aggregates of crystalline grains4 ,calcite is often a secondary product of the lime-bearing silicates in the more basic eruptive rocks4 ,pseudomorphs of calcite after olivine are noteworthy4 ,coarse aggregates of calcite occur in the crystalline schists and contact rocks4 ,calcite is exceedingly plastic to pressure and mechanical deformation may be recognized by curving of the cleavage cracks1 crumpling of the twin lamell and 8wavy0 extinction4 ,calcite is easily attacked and completely dissolved with effervescence by cold dilute acids4 ,h41 #c4 ,sp4 gr41 #b4gb4 ,,dolomite4 ,,anisotropic4 ,,uniaxial4 ,,hexagonal4 ,,composition3 ,ca,mg7,,co#c7b1 when pure ,ca,o #cj4d1 ,mg,o #ba4g1 ,,co#b #dg4h4 ,proportions of ,mg and ,ca vary1 and ,fe and ,mn also occur4 4 ,usual ,appearance in ,sections3 ,in rocks chiefly as crystals1 even dense homogeneous aggregates showing tendency towards crystalline boundaries 7saccharoidal structure74 ,crystals almost always unit rhombohedron 7#ajaa7 with tendency to curved surfaces4 ,index of ,refraction4--n #a4fbb 7 #a4ejc1 #a4fhb1 #j4agi71 a little higher than that of calcite4 ,for variation in appearance of surface with polarized light1 see under calcite4 ,the microscopic characters are similar to those of calcite1 from which it may be distinguished by not being so easily attacked by cold dilute acid 7test can be made on slide with cover off71 by tendency towards crystalline boundaries1 by absence of twin lamell 7or when present parallel to #b,r4 7#bjba71 hence about parallel to shorter diagonals of cleavage rhombs71 and by micro-chemical tests4 ,the distinction at times may be very difficult4 ,,remarks3 ,occurs in sedimentary formations and as crystals in limestone and other rocks4 ,in certain rocks the dolomite crystals may not have a very good 8bond10 and a 8drusy0 structure may also be characteristic of the cavities between dolomite crystals in rocks4 ,only slightly attacked by cold dilute acids1 but if acid is heated it dissolves easily with effervescence4 ,,apatite4 ,,anisotropic4 ,,uniaxial4 ,,hexagonal4 ,,composition3 a4 ,,elongation a4 ,ca#e7;,cl4,f77,,po#d7c4 ,usual ,appearance in ,sections3 ,minute1 slender hexagonal prisms1 cross-sections having regular hexagonal boundaries1 needles1 and grains4 ,figs4 #ad a and #db4 ,illustration3 ,,fig4 #db4--;,apatite1 showing cross fracture1 in nepheline-basalt4 7,from ,cohen47 ,color4--,generally colorless1 seldom bluish or brownish 7only in eruptive rocks74 ,index of ,refraction4--n #a4fce 7 #a4fcd1 #a4fcg71 hence relief more marked than that of the colorless associated minerals4 ,cleavage4--,seldom observed microscopically4 ,parting4--,long columnar crystals generally show a transverse jointing1 so that the pieces may be more or less separated4 ,inclusions4--,gas and fluid may be present4 ,polarized ,light3 ,pleochroism4--,none shown by the colorless crystals1 the colored crystals show stronger absorption parallel to 4 ,crossed ,nicols3 ,double ,refraction4--,weak 7 #j4jjc74 ,interference ,colors4--,the lower first order1 generally grayish-blue or white4 ,extinction4--,as apatite is uniaxial1 basal sections remain dark during rotation of stage4 ,in all other sections extinction is parallel to axis4 ,convergent light3 ,basal sections show a cross1 without rings4 ,optical character 774 ,alteration3 ,does not usually take place1 apatite being found perfectly fresh in decomposed rocks1 which is quite remarkable considering its easy solubility in acids4 ,distinguished from3 7a7 ,,sillimanite and ,,tremolite4--,by weak double refraction and elongation a4 7b7 ,,nephelite4--,by being relatively much smaller and longer than the nephelite crystals1 which are often decomposed4 ,also by higher relief and negative results with gelatinization test4 7c7 ,,zircon4--,by lower relief and much weaker double refraction4 7d7 ,,feldspars 7when granular and undecomposed74--,by higher relief and uniaxial interference figure4 7e7 ,,vesuvianite and ,,zoisite4--,may be only possible by chemical tests4 7f7 ,,corundum4--,see under the latter4 ,,remarks3 ,found in most igneous rocks and crystalline schists4 ,in the eruptive rocks it appears as one of the oldest secretions from the magma1 and hence is often found as inclusions in other minerals1 especially biotite1 hornblende1 etc4 ,apatite is easily soluble in hydrochloric and nitric acids4 ,h41 #d4e to #e4 ,sp4 gr41 #c4ai4 ,on account of its high sp4 gr41 apatite1 in rock powder1 comes down in heavy solutions with the metallic minerals1 and can be separated from them by the use of a magnet4 ,this residue can also be tested for phosphorus in the wet way with ammonium molybdate4 ,,nephelite1#hh ,nepheline1 ,elolite4 ,,anisotropic4 ,,uniaxial4 ,,hexagonal4 ,,composition3 #g,na,al,si,o#d 4 ,na,al7,si,o#c7b1 with partial replacement of ,na by ,k4 ,usual ,appearance in ,sections3 ,nephelite in short hexagonal prisms and grains in the younger volcanic rocks1 hence sections rectangular or hexagonal1 ,fig4 #dc2 elolite allotriomorphic in the older plutonic rocks4 ,color4--,colorless4 ,index of ,refraction4--n #a4eci 7 #a4ech1 #a4edb71 hence no relief and surface smooth4 ,cleavage4--,imperfect1 parallel to prism 7#ajaj7 and base 7#jjja74 ,more marked in elolite than in nephelite1 especially when decomposition has commenced4 ,illustration3 ,,fig4 #dc4--,nephelite sections1 showing zonal inclusions4 7,from ,reinisch47 ,inclusions4--,microscopic needles of augite1 etc41 also fluid and gas4 ,mostly in zones4 ,elolite may be much clouded by inclusions and alteration products4 ,polarized ,light3 ,pleochroism4--,none4 ,crossed ,nicols3 ,double ,refraction4--,very weak 7 #j4jjd71 may only be detected by using a test-plate4 ,interference ,colors4--,the lower first order1 grayish-white1 etc41 a little lower than the feldspar colors4 ,extinction4--,as the mineral is uniaxial1 basal sections remain dark during rotation of stage4 ,in all other sections extinction takes place and is parallel to cleavage lines when these appear4 ,convergent ,light3 ,basal sections show a broad cross1 without rings4 ,optical character 774 ,alteration3 ,takes place easily to fibrous zeolites 7natrolite71 or in certain rocks to mica4 ,distinguished from3 ,,other ,,minerals by gelatinization test and staining with fuchsine4 ,when present in small interstitial individuals 7as is often the case in basalts7 it is very difficult to distinguish without this test2 but it must be remembered that other minerals1 zeolites1 etc41 will also gelatinize4 ,,quartz has stronger double refraction1 rarely shows hexagonal outline1 is always fresh and optically 774 ,,feldspar is biaxial and often shows twinning4 ,,analcite4--,see under the latter4 ,,remarks3 ,nephelite bears the same relation to elolite as sanidine does to orthoclase4 ,it occurs only in the younger volcanic rocks2 with sanidine in phonolite1 with plagioclase in tephrite1 without feldspar in nepheline-basalt1 and with leucite in leucite-basalt4 ,it is not found with primary quartz4 ,elolite occurs with orthoclase in elolite-syenite1 etc4 nephelite and elolite frequently occur with the sodalite group4 ,nephelite gelatinizes with acids4 ,h41 #e4e to #f4 ,sp4 gr41 #b4e to #b4f4 ,,tourmaline1 ,schorl4 ,,anisotropic4 ,,uniaxial4 ,,hexagonal4 ,,composition3 ,uncertain1 a4 ,,elongation a4 ,r#ah;,b#b7,si,o#e7d4 ,r chiefly ,al1 ,k1 ,mn1 ,ca1 ,mg1 ,li4 ,usual ,appearance in ,sections3 ,staff-like individuals1 bunched or in radiating aggregates1 ,fig4 #dd ,b1 or prismatic crystals1 ,fig4 #dd ,a4 ,basal sections may be nine-sided4 ,color4--,varies greatly1 grayish-blue1 brown and green most common4 ,li-tourmaline 7rare in rocks7 is colorless4 ,zonal structure may be indicated by differences in color4 ,illustration3 ,,fig4 #dd4--;,a1 ,tourmaline1 showing strong absorption at right angles to direction of elongation 7,p plane of vibration of polarizer74 ,quartzite1 ,black ,hills1 ,d4 ,b1 ,tourmaline in radiate aggregate4 ,granite1 ,cornwall4 ,index of ,refraction4--n #a4fcc 7precious7 to #a4fgd 7 #a4fbj to #a4fea1 #a4fdj to #a4fhe71 hence relief is marked and surface rough4 ,cleavage4--,not seen in thin sections1 but irregular1 transverse and longitudinal cracks may appear4 ,polarized ,light3 ,pleochroism4--,distinct1 even in light colored varieties1 increasing with the depth of color4 ,the greatest absorption takes place at right angles to the direction of elongation of the crystal1 ,fig4 #dd ,a4 ,the other minerals having this very strong absorption are hornblende1 dark colored mica 7distinguished by cleavage and lamellar form7 and allanite4 ,pleochroic halos may be noticed surrounding inclusions4 ,crossed ,nicols3 ,double ,refraction4--,quite strong 7 #j4jag 7precious7 to #j4jcd74 ,interference ,colors4--,bright upper first or second order1 but may not be noticeable1 due to absorption of parts of the light4 ,extinction4--,as tourmaline is uniaxial1 basal sections remain dark during rotation of stage4 ,in all other sections extinction is parallel to axis4 ,convergent ,light3 ,cross-sections show a sharp cross4 ,optical character 774 ,alteration3 ,does not take place4 ,distinguished from3 7a7 ,,hornblende4--,by absence of cleavage1 and by the fact that the greatest absorption takes place at right angles to the longitudinal axis1 while in hornblende it takes place approximately parallel to the longitudinal axis1 or to the cleavage lines which are parallel to this axis4 7b7 ,,apatite 7when colored74--,by strong absorption at right angles to longitudinal axis4 7c7 ,,corundum4--,see under the latter4 ,in some cases where recognition is difficult1 chemical tests1 to prove presence of boracic acid1 must be made4 ,,remarks3 ,the black schorl is the only primary tourmaline and is found in granitoid rocks4 ,tourmaline in other rocks results from 8fumarole0 action2 hence occurs in pegmatite1 tin and copper veins1 clay deposits1 also 7light colored7 in contact rocks and crystalline schists4 ,the hemimorphic terminations may sometimes be noticed4 ,tourmaline is not acted on by acids4 ,h41 #g to #g4e4 ,sp4 gr41 #c to #c4b4 ,it can be separated from powdered rock by sp4 gr4 solutions combined with magnetic methods4 ,,andalusite ,,anisotropic4 ,,biaxial4 ,,orthorhombic4 ,,composition3 ,al#b,si,o#e4 a4 ,,elongation a4 ,usual ,appearance in ,sections3 ,in short1 rounded1 prismatic crystals1 with almost square cross-section4 ,colorless or at times pale reddish and spotted4 ,index of refraction medium 7n #a4fcg1 #a4fcb1 #a4fdc71 hence relief well marked and surface rough4 ,cleavage1 parallel to almost square prism1 may show4 ,pleochroism only marked in colored varieties1 being reddish 7the direction of elongation or cleavage74 ,carbonaceous inclusions are characteristic1 arranged as in macroscopic specimens 7,chiastolite71 ,fig4 #de4 ,pleochroic halos may surround inclusions4 ,crossed ,nicols3 ,double refraction weak 7 #j4jja74 ,interference colors middle #ast order1 white to yellow4 ,extinction in general parallel to axis in longitudinal sections1 symmetrical in cross-sections4 ,in convergent light ,ax4 pl4 7#jaj71 ,bxa#j4 2 axial angle very large 7#b;,e #ahj72 optical character 774 ,alteration3 ,often takes place to dense aggregate of mica1 when the pseudomorph may be hard to recognize4 ,distinguished from3 7a7 ,,sillimanite by much weaker double refraction1 less elongated crystals and by elongation a 7,sillimanite elong4 c74 7b7 ,,diopside by weaker double refraction1 rhombic cross-section and parallel extinction in longitudinal sections4 ,,remarks3 ,very characteristic of metamorphic schists and of contact zones of clay slates with granite1 etc41 but not found in rocks which have been formed at great pressure4 ,the andalusite grains may often be arranged in divergent or finger-like manner4 ,may also form parallel growths with sillimanite4 ,it is insoluble in hydrochloric acid4 ,h41 #g to #g4e4 ,sp4 gr41 #c4ah4 ,illustration3 ,,fig4 #de4--;,chiastolite1 showing characteristic carbonaceous inclusions4 7,from ,cohen47 ,illustration3 ,,fig4 #df4--,sillimanite aggregate1 showing cross fracture1 in mica schist4 7,from ,cohen47 ,,sillimanite1 ,fibrolite4 ,,anisotropic4 ,,biaxial4 ,,orthorhombic4 ,,composition3 ,al#b,si,o#e4 c c4 ,,elongation c4 ,usual ,appearance in ,sections3 ,long1 slender1 colorless prisms or needles2 often in felt-like aggregates4 ,crystals often bent4 ,index of refraction rather high 7n #a4ffd1 #a4fef1 #a4fgg71 hence relief marked4 ,transverse fractures common1 ,fig4 #df4 ,crossed ,nicols3 ,double refraction rather strong 7 #j4jba74 ,interference colors upper first or lower second order1 red1 purple1 blue1 etc4 ,extinction parallel to prisms4 ,ax4 pl4 7#ajj71 ,bxa4 1 #b;,e #ce to #ee4 ,optical character 774 ,distinguished from3 7a7 ,,apatite by higher order interference colors and by elongation c 7apatite has elongation a74 7b7 ,,tremolite by always parallel extinction and small size of axial angle4 7c7 ,,andalusite1 see under the latter mineral4 ,,remarks3 ,found especially in clay-rich contact rocks1 gneisses and schists1 often occurring with iolite 7cordierite74 ,crystals may appear in bands4 ,it is insoluble in hydrochloric acid4 ,h41 #f to #g4 ,sp4 gr41 #c4bd4 ,,topaz4 ,,anisotropic4 ,,biaxial4 ,,orthorhombic4 ,,composition3 c4 ,al7,al7,o4,f#b77,si,o#d4 ,usual ,appearance in ,sections3 ,colorless crystals of short prismatic habit1 grains or rod-like radiating aggregates4 ,index of refraction about the same as that of calcite 7n #a4fjh to #a4fcb1 #a4fjg to #a4fbi1 #a4fah to #a4fcg71 hence relief medium4 ,cleavage perfect1 parallel to base1 but does not show as many cracks4 ,fluid inclusions abundant4 ,crossed ,nicols3 ,double refraction weak 7 #j4jjh to #j4jaa71 about the same as that of quartz4 ,interference colors middle first order1 white1 yellow1 etc4 ,extinction parallel to cleavage4 ,in convergent light1 ,ax4 pl4 7#jaj71 ,bxa4 1 axial angle large 7#b;,e #hf to #abf72 interference figure obtained from basal sections 7i4 e41 from sections showing no cleavage72 optical character 774 ,alteration3 ,may take place to kaolin or muscovite1 by loss of ,f and taking up of ,h#b,o and alkalies4 ,distinguished from3 7a7 ,,quartz by higher relief1 cleavage and biaxial character4 7b7 ,,sillimanite 7when topaz is in radiating aggregates7 by lower refraction and double refraction4 ,,remarks3 ,common in greisen and all granite rocks containing tin ore4 ,when formed by 8fumarole0 action 7tin veins7 the mineral shows rod-like radiating forms4 ,it is insoluble in hydrochloric acid4 ,h41 #h4 ,sp4 gr41 #c4e4 ,,staurolite4 ,,anisotropic4 ,,biaxial4 ,,orthorhombic4 ,,composition3 ,,elongation 4 ,fe7,al,o7#d7;,al,o,h77,si,o#d7b1 but varying1 may contain ,mg or ,mn4 c4 ,illustration3 ,,fig4 #dg4--,staurolite1 showing twinning at #ij b and #fj c1 also granular quartz inclusions4 7,from ,reinisch47 ,usual ,appearance in ,sections3 ,short1 flat prisms1 which may be twinned at #ij or #fj1 ,fig4 #dg1 or grains4 ,color yellowish to reddish-brown4 ,index of refraction rather high 7n #a4gda1 #a4gcf1 #a4gdf71 hence relief marked and surface rough4 ,cleavage1 both prismatic and pinacoidal1 variable4 ,inclusions of minute quartz grains and carbonaceous matter found in larger crystals1 but not in microscopic crystals4 ,pleochroism distinct but not strong1 showing red c 7direction of elongation74 ,pleochroic halos may surround inclusions4 ,crossed ,nicols3 ,double refraction weak 7 #j4jaj74 ,interference colors middle first order1 white to yellow1 etc4 7about like quartz74 ,extinction in general parallel or symmetrical 7in cross-sections7 to cleavages or crystal outline4 ,in convergent light1 ,ax4 pl4 7#ajj71 ,fig4 #dh4 ,bxa4 1 axial angle large 7#b;,e #ahj72 optical character 774 ,illustration3 ,,fig4 #dh4--,staurolite1 cross-section4 ,alteration3 ,rarely takes place4 ,distinguished from3 ,,titanite1 see under the latter mineral4 ,,remarks3 ,found in metamorphic schists1 associated with cyanite 7disthene71 iolite 7cordierite71 andalusite1 etc4 ,it is one of the minerals produced by thermal metamorphism1 hence found in rocks of granite contact zones4 ,it does not occur in the eruptive rocks or in schists rich in amphibole4 ,staurolite is not acted on by hydrochloric acid4 ,h41 #g to #g4e4 ,sp4 gr41 #c4d to #c4h4 ,,the ,,orthorhombic ,,pyroxenes4 ,enstatite and ,hypersthene4 ,,anisotropic4 ,,biaxial4 ,,orthorhombic4 ,,composition3 7,mg4,fe7,si,o#c4 c4 ,,elongation c4 ,enstatite contains little1 if any1 ,fe4 ,hypersthene contains more ,fe1 its optical characters beginning to show with about #aj per cent4 ,illustration3 ,,fig4 #di4--;,enstatite1 showing columnar or fibrous structure axis4 ,norite1 ,harzburg4 ,usual ,appearance in ,sections3 ,irregularly bounded individuals 7,e7 or rounded prismatic-pyramidal crystals 7,h74 ,columnar or fibrous structure often shows in 7,e71 ,fig4 #di4 ,prism angle about #ib4 ,outline of crystal sections very similar to that of monoclinic pyroxenes4 ,twinning4--,not so common as in monoclinic pyroxenes4 ,parallel growths with monoclinic pyroxene 7diallage7 occur4#hi ,color4--,varies with ,fe per cent41 7,e7 colorless1 7,h7 brownish4 ,index of refraction4--n #a4ffe 7,e7 to #a4gbc 7,h7 7 #a4ffj to #a4gaf1 #a4fgj to #a4gbi7 7about the same as in monoclinic pyroxene71 hence relief marked and surface rough4 ,cleavage4--,variable1 parallel to prism 7angle #ib7 common to all pyroxenes4 ,also cleavage or parting parallel to brachy pinacoid 7#jaj7 7prominent7 and macro pinacoid 7#ajj74hi ,inclusions4--,parallel oriented1 brownish plates and rods1 producing 8schiller0 structure on the principal cleavage faces1 ,fig4 #ae4 ,glass inclusions abundant in 7,h74 ,polarized ,light3 ,pleochroism4--,almost absent in 7,e71 but distinct in 7,h71 increasing with ,fe per cent4 ,the change in color may be very marked1 from brownish-red to greenish 4 ,crossed ,nicols3 ,double ,refraction4--,weak1 much weaker than in the monoclinic pyroxenes1 increasing with ,fe per cent4 7 #j4jaj 7,e7 to #j4jac 7,h747 ,interference ,colors4--,higher first order1 about the same or a little higher than quartz4 ,extinction4--,parallel to cleavages in longitudinal sections1 which are parallel to a or b1 and bisecting angles of intersecting prismatic cleavages in basal sections4 ,convergent ,light3 ,axial plane parallel to brachy pinacoid 7#jaj71hi i4 e41 parallel to best pinacoidal cleavage4 ,bxa4 7,e71 a7,h74 ,axial angles large 7#b;,e #ie to #ahj74 ,optical character for 7,e7771 for 7,h7774 ,on account of weak double refraction the interference figures are not very marked4 ,alteration3 ,takes place to bastite1 serpentine1 etc4 ,distinguished from3 ,the ,,monoclinic ,,pyroxenes and ,,amphiboles4--,see under these species4 ,,remarks3 ,found in the granular rocks of the gabbro-peridotite series1 also in the olivine basalts 7,e72 and in crystals in porphyritic andesite 7,h74 ,these minerals are in general not attacked by acids4 ,h41 #e to #f4 ,sp4 gr41 #c4a to #c4e4 ,bronzite is the name give to the variety containing about #e ,fe and having the characteristic bronzy lustre due to inclusions4 ,bastite 7an alteration product of the orthorhombic pyroxenes poor in ,fe74--,composed of fibers1 often traversed by irregular cracks4 ,color light yellowish or greenish and index of refraction about the same as ,canada balsam4 ,pleochroism faint 7only seen in thick sections71 the greatest absorption taking place parallel to the fibers4 ,double refraction weak and extinction parallel to the fibers4 ,axial angle large and axial plane at right angles to principal cleavage face 7#jaj74 ,the position of the axial plane is the surest distinction between bastite and the orthorhombic pyroxenes4 ,,chrysolite1 ,olivine4 ,,anisotropic4 ,,biaxial4 ,,orthorhombic4 ,,composition3 7,mg4,fe7#b,si,o#d4 ,,elongation a or c4 ,usual ,appearance in ,sections3 ,prismatic crystals or in large angular fragments or grains4 ,longitudinal sections more or less lath-shaped1 with pointed ends1 ,figs4 #ej and #ea1 cross-sections six or eight-sided4 ,outlines of crystals often rounded or corroded4 ,skeleton forms may occur1 and sometimes twinning may be observed4 ,illustration3 ,chrysolite4 ,,fig4 #ej4 ,,fig4 #ea4 ,basal section4 ,macro pinacoid section4 ,color4--,nearly colorless1 may be reddish 7with high ,fe per cent474 ,index of ,refraction4--n #a4fge 7 #a4ffa1 #a4fig71 hence relief marked and surface rough4 ,cleavage4--,parallel to brachy pinacoid 7#jaj71 less distinct parallel to macro pinacoid 7#ajj71 ,fig4 #ej4 ,often only made visible by decomposition4 ,an irregular fracturing occurs1 which increases with alteration into serpentine4 ,inclusions4--,chromite1 opaque earths1 apatite and the brown plates so common in hypersthene2 also glass and slag 7in basaltic rocks7 and fluid 7in peridotites and olivinfels74 ,polarized ,light3 ,pleochroism4--,in general none1 but noticed in the reddish varieties1 when the absorption is a little stronger parallel to 4 ,crossed ,nicols3 ,double ,refraction4--,very strong 7 #j4jcf74 ,interference ,colors4--,rather high in order 7second or third71 higher than the colors of augite4 ,extinction4--,in general parallel to cleavage lines4 ,convergent ,light3 ,axial plane parallel to base 7#jja7 and always at right angles to cleavage cracks1 ,fig4 #ea4 ,bxa4 a4 ,axial angle very large 7#b;,e #ahj74 ,optical character 774 ,alteration3 ,into serpentine very common1#ij producing 8mesh-0 or 8lattice-0 structure 7see under serpentine1 p4 #aad72 also into amphibole1 etc4 ,in certain basaltic rocks the rims of grains may be changed into gthite81 and in certain gabbros the crystals may be surrounded by a radial rim of amphibole4 ,distinguished from3 ,light colored ,,monoclinic ,,pyroxenes4-- ,by the absence of extinction angles1 cleavage 7the intersecting prismatic cleavages of augite being of equal distinctness71 stronger double refraction and by axial plane being parallel to base1 hence always at right angles to best cleavage 7in augite axial plane lies in clino pinacoid1 bisecting angles of intersecting prismatic cleavages74 ,also by gelatinization with acids4 ,,remarks3 ,found only in basic rocks1 as peridotite1 diabase1 gabbro1 norite1 basalts1 etc4 ,chrysolite 7olivine7 is a very brittle mineral and shows under mountain making pressure 8cataclastic0 structure4 ,chromite is a characteristic associated mineral4 ,when not too poor in ,fe1 chrysolite becomes permanently red and pleochroic when strongly heated4 ,chrysolite is decomposed by hydrochloric and sulphuric acids1 with separation of gelatinous silica4 ,h41 #f4e to #g4 ,sp4 gr41 #c4c to #c4d4 ,hyalosiderite 7a more ferruginous chrysolite7 and ,fayalite 7,fe#b,si,o#d7 may be reddish in sections1 and common in the basic porphyritic eruptive rocks4 ,,iolite1 ,cordierite1 ,dichroite4 ,,anisotropic4 ,,biaxial4 ,,orthorhombic4 ,,composition3 a4 ,,elongation a4 ,mg#c7;,al4,fe7#f,si#h,o#bh4 ,usual ,appearance in ,sections3 ,grains1 more rarely crystals of short prismatic habit1 which often form pseudo-hexagonal interpenetration twins4 ,crystals may have edges rounded or corroded4 ,colorless1 but may be bluish4 ,index of refraction a little lower than quartz 7n #a4eci1 #a4ece1 #a4edd71 hence relief low and surface smooth4 ,cleavage very variable1 parallel to brachy pinacoid 7#jaj71 especially noticeable when decomposition has taken place4 ,inclusions of sillimanite1 zircon1 rutile1 etc41 may be seen4 ,pleochroism usually not observed1 but noticed in blue sections 7yellowish white to blue74 ,pleochroic halos 7yellow7 surrounding inclusions common1 see p4 #ei4 ,crossed ,nicols3 ,double refraction weak 7 #j4jji71 like quartz4 ,interference colors middle first order1 white to yellow4 ,extinction in general parallel to cleavage cracks4 ,in convergent light1 ,ax4 pl4 7#ajj71 ,bxa4 2 axial angle large 7hyperbolas only seen without ellipses7 7#b;,e #fd to #aej72 optical character 774 ,alteration3 ,takes place readily1 forming greenish mica-like decomposition products1 the decomposition commencing along the crevices or about the inclusions4 ,distinguished from3 ,,quartz by observation in convergent light 7quartz is uniaxial71 decomposition and pleochroism or pleochroic halos4 ,the section can also be treated with hydrofluosilicic acid1 when the evaporated solution yields characteristic prismatic crystals of magnesium fluosilicate4 ,,remarks3 ,found in gneiss1 hornstone1 granite1 granulite1 etc41 and in some volcanic rocks4 ,it is often associated with garnet1 biotite1 sillimanite1 etc4 ,in a thick section heating to redness makes the pleochroism more distinct4 ,iolite is only slightly acted on by acids4 ,h41 #g to #g4e4 ,sp4 gr41 #b4f4 ,it is hard to make a mechanical separation from quartz1 on account of similarity in sp4 gr4 ,,natrolite4 ,,anisotropic4 ,,biaxial4 ,,orthorhombic4 ,,composition3 c4 ,,elongation c4 ,na#b;,al#b,si#b,o#aj #b;,h#b,o4 ,usual ,appearance in ,sections3 ,aggregates of colorless1 fibrous crystals1 which may have sphrulitic structure1 showing a dark cross between crossed nicols4 ,index of refraction lower than balsam 7n #a4dhc1 #a4dgh1 #a4dij71 hence 7in large crystals7 the surface would appear rather rough4 ,crossed ,nicols3 ,double refraction weak 7 #j4jab74 ,interference colors the middle first order 7yellow1 etc471 a little higher than those of quartz4 ,extinction parallel to fibres4 ,optical character 774 ,,remarks3 ,never a primary mineral in rocks1 but found in igneous rocks filling amygdaloidal cavities1 and also as a very common alteration product of sodalite1 noselite1 nephelite and acid plagioclases4 ,it gelatinizes easily with hydrochloric acid4 ,h41 #e to #e4e4 ,sp4 gr41 #b4b4 ,,other ,,zeolites4 ,,composition3 ,hydrous silicates2 ,al1 ,ca and ,na being the chief bases4 ,usual ,appearance in ,sections3 ,the form depends on the individual mineral species1 but the majority appear in elongated crystals or fibers4 ,they are all colorless and most of them have a small index of refraction1 hence no relief 7prehnite has distinct relief74 ,crossed ,nicols3 ,the double refraction is generally very weak 7between that of nephelite and quartz71 giving very low order interference colors 7prehnite and thomsonite have strong double refraction74 ,,remarks3 ,the zeolites are always secondary minerals in rocks4 ,they gelatinize with hydrochloric acid4 ,,gypsum4 ,,anisotropic4 ,,biaxial4 ,,monoclinic4 ,,composition3 ,ca,s,o#d #b;,h#b,o4 ,usual ,appearance in ,sections3 ,colorless grains or fibers4 ,may be colored1 however1 by inclusions of carbonaceous matter1 iron oxides1 etc4 ,index of refraction about the same as orthoclase 7n #a4ebe1 #a4eba1 #a4eca71 hence no relief and surface smooth4 ,twinning lamell abundant4 ,cleavage parallel to 7#jaj7 gives abundant cracks1 other cleavages may also be noticed4 ,crossed ,nicols3 ,double refraction weak 7 #j4jaj71 the same as quartz4 ,interference colors middle first order1 white to yellow4 ,extinction parallel to most perfect cleavage cracks in sections parallel to b axis2 large extinction angles noticed with reference to less perfect cleavages4 ,in convergent light1 ,ax4 pl4 7#jaj71 i4e41 to most distinct cleavages2 ,bxa4 7c7 #ed front2 #b;,e #ajd2 optical character 774 ,as the characters of gypsum are not always very marked it may be necessary to employ micro-chemical tests4 ,,remarks3 ,forms a rock by itself1 often associated with rock salt4 ,it also occurs as an alteration product of anhydrite4 ,gypsum is soluble in hydrochloric acid4 ,h41 #a4e to #b4 ,sp4 gr41 #b4b to #b4d4 ,,monoclinic ,,pyroxenes1 ,augite1 etc4 ,including the monoclinic minerals of the ,pyroxene ,group1 which show distinctly the characteristic cleavage parallel to an almost right-angled prism4 ,,anisotropic4 ,,biaxial4 ,,monoclinic4 ,,elongation c4#ia ,,composition3 ,r,si,o#c1 ,r ,ca1 ,mg1 ,mn1 ,fe1 ,al chiefly1 with the ,ca predominating over the ,mg4 ,usual ,appearance in ,sections3 ,both in crystals and more or less irregular grains1 ,figs4 #d and #ab1 the habit varying with the chemical composition as follows3 ,diopside 7,ca1 ,mg varieties71 long columnar crystals and grains4 ,augite 7ditto1 but containing also ,al and ,fe71 short prismatic crystals and grains4 ,diallage1 granular or lamellar 7 7#ajj771 may show fibrous structure 4 ,prism angle #hg #jf 7important in cross-sections74 ,sections of crystals nearly at right angles to the vertical axis are octagonal or square with truncated corners1 ,figs4 #d and #ec1 while those parallel to the axis are lath-shaped4 ,pyroxene also occurs in skeleton crystals and acicular microlites in eruptive rocks4 ,illustration3 ,,fig4 #eb4--;,augite1 section parallel to axis showing prismatic cleavage1 in leucite-basal4 7,from ,cohen47 ,zonal structure 7especially in augites7 may be marked by differences in color or extinction1 and in some basalts the crystals have the 8hour-glass0 structure4 ,twinning4--,common1 usually the twinning plane being the ortho pinacoid 7#ajj74 ,twin lamell may be noticed4 ,intergrowths occur with orthorhombic pyroxene and amphibole4 ,color4--,from almost colorless through green 7diopsides1 ,na pyroxenes1 etc47 to brown 7augites72 the red to brownish-red color of certain augites has been considered due to manganese4 ,yellow color very rare4 ,index of ,refraction4--n #a4fh to #a4gb 7 #a4fga to #a4gjf1 #a4gjj to #a4gbh71 hence relief high and surface rough4 ,cleavage4--,more or less perfect parallel to prism of #hg #jf4 ,cleavage cracks distinct and numerous1 but not generally running uninterruptedly through crystal1 ,figs4 #ab and #eb4 ,cleavage not so perfect as that of amphibole4 ,parting4--,diallage and diopside have distinct parting parallel to ortho pinacoid 7#ajj71 ,fig4 #ec4 ,some crystals may show parting parallel to base 7#jja74 ,inclusions4--,tabular microscopic interpositions1 similar to those in bronzite1 may occur in diallage4 ,the iron ores1 apatite1 etc41 may occur in augite4 ,polarized ,light3 ,illustration3 ,,fig4 #ec4--;,diallage1 cross-section4 ,pleochroism4--,usually not noticed1 and in general only appearing as different shades of the same color4 ,in some cases 7diallage1 fassaite and ,na rich augite7 well marked1 a and c green to yellowish green and b brownish to reddish-brown2 hence pleochroism not intense in sections showing extinction angles4 ,when ,ti is present1 violet parallel to b4 ,crossed ,nicols3 ,double ,refraction4--,strong 7 #j4jbb to #j4jbi71 being stronger in the pale or colorless pyroxenes4 ,interference ,colors4--,second order1 hence always bright tints4 ,extinction4--,symmetrical in sections 7through b axis7 showing intersecting cleavage lines1 in such cases bisecting the angles of the cleavage4 ,in sections showing parallel cleavage lines1 only parallel in ortho pinacoid 7#ajj7 sections1 in all other sections an extinction angle being observed4 ,the maximum extinction angle is large1 lies in the obtuse angle1 varies with the chemical composition from #cf #cj to #ed1 and is only obtained when the section of the crystal is parallel to the clino pinacoid 7#jaj71 ,fig4 #ed1 varying from this angle to #j1 when the section is parallel to the ortho pinacoid 7#ajj74 ,in ,ti and ,na pyroxenes the inclined dispersion is so great that extinctions are not sharp1 but instead a change takes place in the interference color from bluish to brownish4 ,convergent ,light3 ,axial plane parallel to clino pinacoid 7#jaj74 ,fig4 #ed4 ,a cleavage flake parallel to ortho pinacoid 7#ajj7 shows the emergence of an optic axis 7orthorhombic pyroxene parallel to best pinacoidal cleavage would not show figure74 ,bxa47c7 #cf to #ed front4 ,axial angles large 7#b;,e #gj to #aab74 ,optical character 774 ,the interference figures are distinct on account of the strong double refraction4 ,alteration3 ,may take place to chlorite1 serpentine or amphibole 7uralitization#ib71 depending on the chemical composition and the conditions producing the change4 ,illustration3 ,,fig4 #ed4--;,diopside1 clino pinacoid section4 ,distinguished from3 7a7 ,,orthorhombic ,,pyroxenes4--,by extinction angle1 the orthorhombic pyroxenes having always parallel or symmetrical extinction in sections parallel to a1 b1 or c1 and by higher order interference colors4 ,also from hypersthene by absence of1 or much fainter1 pleochroism4 ,diallage and bronzite might be confused on account of pronounced pinacoidal parting1 fibrous structure and inclusions2 but may be distinguished by the presence or absence of extinction angles and also by the position of the optic axes relative to the best cleavage plates4 7b7 ,,amphibole4--,see under amphibole4 7c7 ,,epidote and ,,chrysolite 7,olivine74 ,when light colored and granular1 by examination in convergent light4 ,the plane of the optic axes is parallel to the clino pinacoid 7#jaj71 hence to the longitudinal axis and prismatic cleavage cracks1 while in epidote it is at right angles to these directions and in chrysolite parallel to the base4 ,also yellow color is common in epidote but rare in pyroxene4 ,,remarks3 ,next to the feldspars pryoxene is the most common constituent of the igneous rocks4 ,diopside and fassaite 7green7 are found in contact rocks2 also1 what appear to be the same pyroxenes1 in many eruptive rocks1 as andesites1 monzonites1 etc4 ,malacolite 7light green7 is found in amphibolites and eclogites1 where it may be associated with a greenish amphibole 7smaragdite74 ,diallage 7bladed and twinned7 occurs in gabbros and pyroxenites4 ,common augite 7brown7 is found in the remaining basic eruptive rocks4 ,in the schists the pyroxene is colorless4 ,finally augite occurs as a secondary product resulting from the 8magmatic resorption0 of hornblende and biotite4 ,chemical corrosion and mechanical deformation may occur4 ,the green and brown augites when heated to redness on platinum foil may become red in color4 ,in general the pyroxenes are not attacked by acids4 ,h41 #e to #f4 ,sp4 gr41 #c4c to #c4e4 ,the sp4 gr4 of the pyroxenes is considerably higher than that of the amphiboles of similar composition1 hence mechanical separations are possible4 ,acmite 7girine7 7,na pyroxenes74--,occur in green or brown1 elongated prismatic crystals1 often not very transparent and with marked pleochroism 7like amphibole74 ,zonal coloring is common4 ,when zonally intergrown with pyroxene the outer zone is girine4 ,the elongation is a 7distinction from amphibole whose elongation is c74 ,the index of refraction is higher than in the other pyroxenes 7n #a4gib1 #a4gfc1 #a4hac7 and the double refraction stronger 7 #j4jej74 ,the extinction angle is small 7#e7 and the optical character 7--74 ,the term girine-augite may be used to describe a soda1 pleochroic augite with a large extinction angle4 ,these pyroxenes are only found in the eruptive rocks rich in alkalies1 as elolite-syenite1 phonolite1 certain trachytes1 etc42 hence are associated with elolite1 sodalite1 leucite1 etc4 ,the small1 second generation1 crystals1 in the ground mass of a rock1 are always the richest in ,na of the pyroxenes in that rock4 ,,amphibole1 ,hornblende1 etc4 ,,anisotropic4 ,,biaxial4 ,,monoclinic4 ,,elongation c4 ,,composition3 ,r,si,o#c4 ,r ,mg1 ,ca1 ,fe chiefly also may contain ,al1 ,na1 ,mn4 ,the ,mg predominates over the ,ca4 ,illustration3 ,,fig4 #ee4--;,hornblende1 showing twinning between crossed nicols1 in amphibole-biotite-granite4 7,from ,cohen47 ,usual ,appearance in ,sections3 ,both in crystals and more or less irregular grains1 often fibrous1 ,figs4 #ee and #ef1 the habit varying with the chemical composition as follows3 ,tremolite 7,mg#c;,ca7 and ,actinolite 77,mg,fe7#c;,ca varieties71 in long columnar to needle-like individuals1 with no terminal planes or with frayed out ends4 ,may be in dense aggregates4 ,pargasite in well developed crystals4 ,common green ,hornblende 7aluminous varieties7 in crystals1 compact grains or shreds4 ,basaltic ,hornblende 7iron rich1 aluminous varieties7 in prismatic crystals of varying length1 which may often show 8magmatic resorption0 7to augite and magnetite7 around outer zone or throughout whole crystal4 ,illustration3 ,,fig4 #ef4--;,hornblende1 section parallel to axis1 showing prismatic cleavage1 in hornblende-diorite4 7,from ,cohen47 ,crystals are simple in form of prismatic habit1 with prism angle #abd #cj4 ,cross-sections are acutely rhombic1 generally with acute angles truncated1 hence six-sided 7pyroxene being eight-sided74 ,longitudinal sections are lath-shaped and fibrous structure may be noticed4 ,skeleton crystals may also occur1 being very fine in certain pitchstones4 ,zonal structure and parallel growth may be noticed in the amphiboles4 ,twinning4--,frequent1 parallel to ortho pinacoid 7#ajj74 ,twins dual1 less often multiple1 ,fig4 #ee4 ,intergrowths with pyroxene and biotite occur4 ,color4--,from colorless 7tremolite71 through green 7actinolite1 pargasite and hornblende7 to brown 7basaltic hornblende74 ,yellow in some varieties and bluish in the soda varieties4 ,index of ,refraction4--n #a4fba to #a4fda 7 #a4fjg to #a4fbi1 #a4fcd to #a4fec7 7#a4gai1 in the basaltic hornblende71 hence relief distinct and surface rough4 ,cleavage4--,perfect1 parallel to prism of #abd #cj4 ,generally appears in thin sections as sharp cracks crowded close together1 ,figs4 #ef and #eg4 ,more perfect than in pyroxene4 ,some of the long prisms 7actinolite and tremolite7 may show transverse parting4 ,inclusions4--,the iron ores1 apatite1 etc41 may be found in hornblende4 ,illustration3 ,,fig4 #eg4--;,hornblende1 cross-section4 ,polarized ,light3 ,pleochroism4--,all colored amphiboles show pleochroism1 which in general is stronger the darker the color of the variety 7actinolite and pargasite show but little74 ,the absorption is very marked in the hornblendes1 being greatest in the general direction of the cleavage lines in longitudinal sections4 ,marked differences in absorption are also characteristic of the mineral species biotite1 tourmaline and allanite4 ,pleochroic halos 7brownish7 surrounding inclusions may be noticed4 ,crossed ,nicols3 ,double ,refraction4--,quite strong1 but a little weaker than in pyroxene 7- #j4jai to #j4jbg74 ,ferruginous basaltic hornblende has strong double refraction 7- #j4jgb74 ,illustration3 ,,fig4 #eh4--;,actinolite1 clino pinacoid section4 ,interference ,colors4--,second order1 hence bright tints1 but in darker colored varieties not so noticeable as in pyroxenes1 due to the stronger absorption of parts of the light4 ,the colors of basaltic hornblende are so high that they show no bright tints4 ,extinction4--,always symmetrical in sections 7through b axis7 showing intersecting cleavage lines1 in such cases bisecting the angles of the cleavage4 ,in sections showing parallel cleavage lines1 only parallel in ortho pinacoid 7#ajj7 sections1 in all other sections an extinction angle being observed4 ,the maximum extinction angle lies in the acute angle and is much smaller than in pyroxene1 varying with the chemical composition from #j-bj4 ,in hornblende1 actinolite and tremolite #ab-bj1 ,fig4 #eh2 in the basaltic hornblende #j-aj4 ,the maximum extinction angle is only obtained when the section of the crystal is parallel to the clino pinacoid 7#jaj71 varying from this angle to #j1 when the section is parallel to the ortho pinacoid 7#ajj74 ,convergent ,light3 ,axial plane parallel to clino pinacoid 7#aaj71 ,fig4 #eh4 ,bxo7c7 #j-bj behind4 ,axial angles large 7#b;,e #gg to #ahj74 ,optical character 774 ,pargasite is 774 ,alteration3 ,may take place to chlorite1 talc1 serpentine1 asbestus1 etc41 depending on the chemical composition4 ,amphibole frays out and becomes fibrous during alteration1 and may also lose color4 ,distinguished from3 7a7 ,,pyroxene4--,by usually much stronger pleochroism in the colored varieties1 and by cleavage and extinction angle4 ,in pyroxene the cleavage 7parallel to prism of #hg #jf7 is less perfect2 and the extinction angle is much larger1 varying from #cf to #ed4 7b7 ,,biotite4--,by the extinction in the mica being always about parallel to the cleavage4 ,both have strong absorption1 but biotite shows very slight pleochroism in sections parallel to the cleavage1 and has only the one cleavage parallel to the base4 ,also the biotite has lower index of refraction and generally shows uniaxial interference figure4 ,colorless tremolite may be distinguished from muscovite and talc by extinction angles1 relief and lower order interference colors4 7c7 ,,tourmaline4--,by presence of cleavage1 and by the fact that absorption is most marked about parallel to the elongation 7also parallel to cleavage lines71 while in tourmaline the absorption is strongest at right angles to the elongation4 7d7 ,the ,,orthorhombic ,,pyroxenes4--,by extinction angles1 the latter having parallel extinction in all sections parallel to a1 b and c1 and by prismatic cleavage of #abd #cj4 ,pleochroism is strong in the colored varieties of both species1 but in amphibole it appears more generally as a variation of the same color2 while in hypersthene a change in color is often noticed1 from brownish-red to greenish parallel to axis4 7e7 ,,sillimanite and ,,cyanite4--,see under the latter4 ,,remarks3 ,amphibole comes next to pyroxene in importance and distribution of the dark colored ferruginous rock-forming minerals4 ,as a rule it occurs in rocks with a large percentage of ,si,o#b1 associated with quartz and orthoclase2 while augite generally occurs in rocks of a basic nature1 associated with plagioclase and little or no free ,si,o#b4 ,furthermore amphibole contains hydroxyl and is therefore naturally found in the deep eruptive rocks2 its place being taken by augite in the effusives4 ,by application of heat hornblende changes to augite1 while hydrochemical processes bring about the opposite result 8uralitization40 ,tremolite and actinolite are found in contact rocks and crystalline schists1 also as a result of the alteration of olivine into serpentine4 ,pargasite occurs in contact rocks4 ,common green hornblende is found in the plutonic rocks 7,na poor and ,si,o#b rich71 also in contact rocks and crystalline schists 7amphibolites74 ,brown hornblende replaces the green variety in the basic plutonic rocks4 ,basaltic hornblende is found in many effusive rocks4 ,the hornblende crystals in eruptive rocks1 being among the first formed constituents1 have often suffered subsequent corrosion by the magma1 giving rise to the dark border already mentioned4 ,the brown primary hornblende in some rocks may be changed by a process analogous to 8uralitization0 into a green1 reed-like hornblende4 ,mechanical deformations are found in massive and schistose rocks4 ,light green amphiboles1 with weak pleochroism1 may often be colored intensely reddish-brown and made strongly pleochroic by heating to redness on platinum foil4 ,in general the amphiboles are not affected by acids4 ,h41 #e to #f4 ,sp4 gr41 #b4i to #c4c4 ,glaucophane1 ,arfvedsonite1 etc4 7,na rich amphiboles74--,occur blue to bluish-green in color1 with pleochroism and weaker double refraction than the other amphiboles4 ,extinction angles vary from #d-f 7glaucophane7 to #ad 7arfvedsonite74 ,they are found in contact rocks1 crystalline schists1 eclogite1 etc4 ,for the rarer and less known members of the amphibole group1 resource should be had to more elaborate works4 ,uralite4--,pyroxene altered to amphibole1 having the outward crystal form of pyroxene and the physical characters and cleavage of amphibole4 ,the change usually commences on the surface and the uralite does not form a single compact crystal1 but consists of numerous slender columns parallel to one another4 ,these little columns or fibers have their c and b axes parallel to the positions of these axes in the parent mineral4 ,the color is green and the pleochroism weak to strong4 ,this change is called 8uralitization0 and results from hydrochemical processes4 ,when the alteration is not complete1 portions of the original pyroxene may be left1 having all the characteristic optical properties of this latter mineral4 ,anthophyllite1 the orthorhombic amphibole1 with always parallel extinction1 is sometimes found in colorless to brownish1 blade- to rod-like aggregates in crystalline schists and serpentine4 ,,mica ,,group4 ,,anisotropic4 ,,biaxial4 ,,monoclinic4 ,may appear hexagonal or orthorhombic4 ,,composition3 ,,elongation 7 cleavage7 c4 ,biotite 7black or ferro-magnesium mica7 7,h4,k7#b7,mg4,fe7#b;,al#b7,si,o#d7c1 approx4 ,phlogopite a magnesium mica1 near biotite1 but containing little ,fe4 ,illustration3 ,,fig4 #ei4--,mica4 ,a1 biotite1 showing hexagonal cross-section and zonal markings4 ,minette1 ,freiburg4 ,b1 ,biotite1 showing strong absorption parallel to cleavage and also zonal marking 7,p plane of vibration of polarizer74 ,minette1 ,cumberland4 ,c1 ,muscovite in bent shreds in gneiss4 ,muscovite 7white or potash mica7 ,h#b7,k4,na7,al#c7,si,o#d7c1 with some replacement by ,mg or ,fe4 ,usual ,appearance in ,sections3 ,scales1 which may be notched or jagged1 with lateral sections lath-shaped2 or shreds1 ,fig4 #ei ,c4 ,when distinctly crystallized 7magnesium micas7 the thin hexagonal plates have plane angles of #abj1 ,fig4 #ei ,a4 ,phlogopite crystals may be extended in direction of axis4 ,zonal structure not uncommon in the magnesium micas1 ,fig4 #ei ,a1 which may also have dark iron ore border like hornblende4 ,twinning4--,common1 generally parallel to base2 seen in sections showing cleavage by variations in extinction1 in basal sections by distorted interference figures4 ,micas of different kinds often associated together in parallel position1 also intergrown with hornblende1 pyroxene1 chlorite and quartz4 ,color4--,depends on chemical composition4 ,biotites1 brown1 green or red to almost opaque4 ,phlogopites1 colorless or yellowish4 ,muscovites colorless4 ,index of ,refraction4--n #a4efd to #a4fai 7 #a4eda to #a4ehj1 #a4ege to #a4fch71 hence somewhat marked relief and surface varies in appearance from slightly rough to fairly rough4 ,in polarized light the surface appears roughest when the cleavage cracks are parallel to the plane of the polarizer4 ,biotite has more marked relief4 ,cleavage4--,very perfect1 parallel to base 7#jja71 ,fig4 #fj4 ,basal sections show no cleavage1 but all other sections show many sharp1 parallel cleavage cracks4 ,illustration3 ,,fig4 #fj4--;,biotite1 showing basal cleavage1 in biotite-granite4 7,from ,cohen74 ,for percussion and pressure figures1 see reference given below4#ic ,inclusions4--,may be arranged parallel to lines of pressure figure4 ,rutile needles1 tourmaline1 apatite1 etc41 common in magnesium mica4 ,zircon inclusions often surrounded by pleochroic halos4 ,polarized ,light3 ,pleochroism4--,varies with the color1 being very marked in the colored varieties 7from pale yellow to chestnut-brown or black74 ,the strong absorption1 about parallel to the cleavage lines1 is very characteristic of the colored mica1 ,fig4 #ei ,b4 ,strong absorption is also noticed in hornblende1 tourmaline and allanite4 ,absorption may even be noticed around inclusions 7pleochroic halos7 in colorless1 non-pleochroic micas4 ,cleavage plates of biotite are not pleochroic unless the axial angle is large4 ,crossed ,nicols3 ,double ,refraction4--,very strong 7 #j4jcd to #j4jeh74 ,interference ,colors4--,high order 7third74 ,may be very bright in thin sections of the colorless micas1 and at times be so high in order as not to show any marked color tints4 ,may not be noticeable in sections of the colored varieties1 due to absorption of parts of the light4 ,extinction4--,about parallel to cleavage lines4 ,very small extinction angles may be noticed in biotites4 ,basal sections of biotite 7the approximately hexagonal mica7 usually appear isotropic4 ,mottled appearance 7like 8,birds-eye0 maple7 characteristic1 caused by distortion of the flexible lamin during grinding4 ,most noticeable in sections inclined to cleavage and near position of extinction4 ,convergent ,light3 ,axial plane#id and ,bxa1 practically at right angles to basal cleavage2 therefore cleavage plates always show well defined interference figures1 generally biaxial in character4 ,the axial angles vary greatly1 being usually small for biotite and phlogopite 7may appear uniaxial7 and large for muscovite 7#b;,e #ee to #ij74 ,optical character for all micas 774 ,alteration3 ,biotites decompose quite easily1 lose color and may become completely bleached1 which appears to be due to a leaching out of the iron4 ,may also alter to green chlorite1 with a fraying out of the mica and a change to chloritic structure4 ,phlogopites may alter to fibrous1 scaly masses1 apparently chiefly talc4 8,sagenite0 webs of rutile may accompany the alteration4 ,muscovites are characterized by their freshness1 and do not seem to suffer from weathering4 ,distinguished from3 7a7 ,,hornblende4--,magnesium mica has extinction about parallel to the cleavage1 while hornblende may have extinction angles of from #j to #bj4 ,both have strong absorption1 but biotite shows very slight pleochroism in basal sections1 which also give approximately uniaxial interference figures in convergent light4 7b7 ,,tourmaline4--,magnesium mica shreds show absorption parallel to elongation1 while in tourmaline the absorption is at right angles to elongation4 ,there is also an absence of cleavage in tourmaline4 7c7 ,,chlorite4--,by strong double refraction1 the very high order colors1 however1 being often not noticed4 ,chlorite also shows aggregate structure and is almost always greenish in color4 7d7 ,,talc4--,white mica by large axial angle of scales in convergent light and by micro-chemical tests4 ,the distinction may be very difficult4 ,,remarks3 ,muscovite is a rare primary mineral in eruptive rocks1 except in two-mica granite1 etc4 ,as a secondary mineral it occurs in dense scaly aggregates or as pseudomorphs after feldspar1 nephelite1 etc4 ,it is frequent in crystalline schists and is probably also the mica in amphibolite and eclogite4 ,phlogopite is found chiefly in contact metamorphic limestone2 and may be distinguished from muscovite by nearly uniaxial character and less sharp cleavage4 ,biotite is much more widely distributed1 occurring especially in eruptive rocks1 crystalline schists and contact rocks4 ,chemical corrosion occurs in original biotite of porphyritic rocks1 producing a 8resorption border0 of augite and magnetite4 ,mechanical deformations1 producing bending and slipping along 8gliding0 planes 7oblique to cleavage71 are common to all varieties of mica and may produce change to chlorite4 ,the muscovites1 together with the feldspars1 are the most characteristic minerals of dynamo metamorphic origin4 ,biotites and phlogopites are attacked by sulphuric acid at high temperatures4 ,muscovite is but slightly attacked by acids4 ,h41 #b to #c4 ,sp4 gr41 #b4g to #c4b4 ,the specific gravity separation between the micas is difficult on account of the scaly nature of the minerals4 ,other micas occur1 some being alteration products of those already described4 ,among these may be mentioned3 ,lithia ,mica4--,both light and dark colored1 occurring in granitic rocks and often only distinguished chemically from muscovite and biotite4 ,damourite 7,sericite7 7hydrous ,k mica74--,a secondary product usually in colorless1 fine scaly aggregates in phillites1 sericite-schists1 etc4 ,,chlorite ,,group4 ,embracing the members of the ,chlorite ,group1 commonly occurring in rocks4 ,,anisotropic4 ,,biaxial4 ,,monoclinic4 ,the minerals of this group usually appear uniaxial1 and crystallize in part with hexagonal symmetry4 ,,composition3 ,may be considered as isomorphous mixtures of ,h#d7,mg,fe7#c,si#b,o#i and ,h#d7,mg,fe7#b7;,al,fe7#b,si,o#i 7,rosenbusch74 ,usual ,appearance in ,sections3 ,minute1 scaly aggregates1 which may incline to radial grouping2 or in minute grains as a pigment 7veridite7 in other minerals4 ,twinning4--,may be seen as in mica4 ,color4--,generally green1 varying from greenish white to dark green1 rarely colorless or red4 ,index of ,refraction4--n #a4efg to #a4ehi 7 #a4efj to #a4ehe1 #a4ega to #a4eif71 hence no marked relief and only slightly rough surface4 ,cleavage4--,like mica1 very perfect2 parallel to flat face1 which is considered to be the basal plane4 ,this cleavage may not be noticed1 especially in secondary chlorite in rocks4 ,polarized ,light3 ,pleochroism4--,in green and yellow tints 7green cleavage71 being more marked in dark colored varieties4 ,basal sections are non-pleochroic1 the mineral being practically uniaxial4 ,pleochroic halos may be seen4 ,crossed ,nicols3 ,double ,refraction4--,generally very weak 7 #j4jja to #j4jaa74 ,interference ,colors4--,very low first order1 gray or white1 at times scarcely noticed4 ,anomalous colors1 however1 often seen 7deep blue or brown47 ,extinction4--,plates parallel to cleavage generally appear isotropic or only show faint color4 ,in other sections extinction is apparently parallel to the cleavage in the uniaxial type1 but extinction angles may be noticed when type is biaxial4 ,complete extinction may not be noticed1 due to aggregate structure4 ,convergent ,light3 ,plates parallel to cleavage show1 at times1 an indistinct interference cross1 which may open into two hyperbolas1 indicating biaxial nature of crystallization4 ,ax4 pl4 7#jaj72 ,bxa4 #j to #ae2 #b;,e variable4 ,optical character 774 ,distinguished from3 ,,serpentine4--,by general green color 7serpentine1 with exception of ,fe rich variety1 is colorless71 pleochroism and frequent anomalous interference colors2 but the distinction between these two minerals may be very difficult4 ,chlorite may resemble decomposed or green mica 7mica has1 however1 strong double refraction74 ,the different species in the chlorite group cannot usually be distinguished in rocks4 ,,remarks3 ,the chlorites are essentially secondary minerals1 derived from the aluminous silicates1 biotite1 augite1 garnet1 feldspar1 etc4 ,they are found abundantly in chlorite-schists1 contact rocks1 etc41 and as pigment 7veridite7 in altered eruptive rocks4 ,may occur as a primary constituent of eruptive rocks1 often in parallel growth with biotite4 ,chlorides are acted on by hot hydrochloric acid1 and decomposed easily by sulphuric acid4 ,h41 #b to #c4 ,sp4 gr41 #b4f to #b4if4 ,a thin section heated to redness on platinum foil loses water and becomes opaque4 ,ferruginous varieties are turned reddish-brown to black by heating 7serpentine1 as it contains less iron1 may give negative results with this test74 ,delessite4--,found in sphrulites1 filling cavities in amygdaloidal basic rocks1 and in pseudomorphs4 ,it appears to be much altered to other minerals4 ,,talc4 ,,anisotropic4 ,,monoclinic 7,pseudo-,hexagonal74 ,,composition3 ,,elongation c4 ,h#b,mg#c7,si,o#c7d4 ,usual ,appearance in ,sections3 ,in fine scaly1 colorless aggregates4 ,sections1 cutting across the scales1 would show rod-like forms4 ,index of refraction only a little higher than balsam 7n #a4egb1 #a4eci1 #a4ehi71 hence no marked relief and only slightly rough surface4 ,cleavage perfect parallel to base1 like mica4 ,crossed ,nicols3 ,double refraction very strong 7 #j4jej74 ,interference colors third order1 like muscovite4 ,extinction parallel to basal cleavage lines4 ,in convergent light1 ,ax4 pl4 7#ajj71 ,bxa4 2 axial angle small 7#b;,e small72 optical character 774 ,distinguished from3 ,,muscovite 7with which it is easily confused7 by micro-chemical tests1 proving absence of alkalies and ,al2 and often by the more aggregate structure of the talc4 ,also by smaller axial angle of scales in convergent light4 ,,remarks3 ,found mainly in metamorphic schists1 etc41 always as a secondary product4 ,it is insoluble in hydrochloric acid4 ,h41 #a to #a4e4 ,sp4 gr41 #b4f to #b4h4 ,,epidote4 ,,anisotropic4 ,,biaxial4 ,,monoclinic4 ,,elongation a or c4 ,,composition3 ,ca#b;,al#b7;,al,o,h77,si,o#d7c1 with some ,fe replacing ,al4 ,usual ,appearance in ,sections3 ,columnar to thick tabular crystals1 more or less elongated parallel to ortho axis b1 ,fig4 #fa1 or in granular aggregates4 ,illustration3 ,epidote4 ,,fig4 #fa4 ,,fig4 #fb4 ,ortho pinacoid section4 ,clino pinacoid section4 ,twinning4--,may occur but rarely noticed4 ,irregular interpenetrations common with other members of the group2 also parallel growths4 ,color4--,colorless to yellowish 7,fe poor7 or yellow to greenish to yellow-brown 7,fe rich74 ,index of ,refraction4--n #a4gea but may be lower 7 #a4gca1 #a4gfh71 hence relief high and surface rough4 ,cleavage4--,parallel to base 7#jja71 ,figs4 #fa and #fb1 imperfect parallel to ortho pinacoid 7#ajj74 ,basal cleavage cracks not very numerous and appear parallel to general direction of elongation4 ,polarized light3 ,pleochroism4--,varies with the color1 being faint in the light colored varieties1 but strong when the color is marked 7,fe rich74 ,crossed ,nicols3 ,double ,refraction4--,variable1 often very strong 7 #j4jcg74 ,interference ,colors4--,variable1 often high 7third7 orders4 ,intergrowths with other members of the group are clearly shown by the 8flecked0 interference colors4 ,extinction4--,parallel to cleavage in sections parallel to b axis4 ,in other sections extinction angles vary1 see ,fig4 #fb4 ,convergent ,light3 ,axial plane 7#jaj71 i4 e41 at right angles to the elongation of crystal and cleavage cracks1 ,fig4 #fa4 ,bxa4-7a7 #c behind4 ,basal cleavage flakes show the almost emergence of an optic axis4 ,axial angles very large 7#b;,e #ahj74 ,optical character 774 ,alteration3 ,does not take place readily4 ,distinguished from3 ,light colored ,,monoclinic ,,pyroxene4--,by having plane of optic axes at right angles to cleavage cracks and direction of elongation2 while in pyroxene plane of optic axes is parallel to parallel prismatic cleavage cracks or bisects the angle between intersecting cracks4 ,furthermore the yellow color is rare in pyroxene4 ,,remarks3 ,epidote is essentially a secondary mineral1 resulting from the alteration of the feldspars and the ferro-magnesium silicates4 ,it is found in crystalline schists 7especially those containing hornblende71 gneiss1 gabbro1 diorite1 diabase1 lime-silicate hornstones1 contact rocks1 etc4 ,epidote is partially decomposed by hydrochloric acid4 ,the ,fe rich epidote can be changed to an intense color by 8glowing0 in the air4 ,h41 #f to #g4 ,sp4 gr41 #c4cb to #c4de4 ,piedmontite 7containing ,mn74--,red in color4 ,very pleochroic1 red to yellow4 ,found in crystalline schists1 the porphyrite of ,scotland1 the famous 8,porfido rosso antico0 of ,egypt and in certain ,japanese mica schists4 ,,zoisite4 ,essentially orthorhombic8 members of ,epidote group4#ie ,,composition3 ,like epidote but without any ,fe4 ,usual ,appearance in ,sections3 ,similar to epidote or in columnar aggregates4 ,often intergrown with epidote4 ,distinguished from epidote by general absence of color 7colorless to yellowish7 and pleochroism2 by slightly lower refractive index 7n #a4fii to #a4gbj7 and by much weaker double refraction 7 #j4jje and less74 ,the interference colors are very low order1 gray to white1 but anomalous colors are often seen 7yellow or prussian blue74 ,extinction is in general parallel to pinacoidal cleavage cracks 7except in clinozoisite74 ,the plane of the optic axes may be parallel or at right angles to cleavage cracks1 and the optical character is 774 ,,remarks3 ,generally a secondary mineral4 ,found in crystalline schists1 amphibolites1 contact rocks1 eclogite1 etc41 and in 8saussurite40 ,may be hard to distinguish from vesuvianite and apatite4 ,,allanite1 ,orthite4 ,monoclinic member of ,epidote group4 ,,composition3 ,like epidote but containing cerium4 ,usual ,appearance in ,sections3 ,similar to epidote in form2 but distinguished by brown color 7may be also almost colorless71 strong pleochroism and absorption1 and medium to weak double refraction 7 #j4jjb to #j4jcj74 ,lamellar twinning clearly seen on account of oblique extinction4 ,when included in hornblende and mica it is surrounded by pleochroic halos4 n #a4gh about4 ,optical character 774 ,usually perfectly fresh4 ,,remarks3 ,found as an accessory mineral in ,si,o#b rich eruptive rocks and connected crystalline schists1 and 7light colored7 in amphibolite and eclogite4 ,,titanite1 ,sphene4 ,,anisotropic4 ,,biaxial4 ,,monoclinic4 ,,composition3 ,ca,ti,si,o#e4 ,,elongation a4#if ,usual ,appearance in ,sections3 ,wedge-shaped crystals 7in ,na rich rocks1 more prismatically developed72 grains1 which may be elongated2 and aggregates of small rounded particles1 which appear nearly opaque4 ,sections of crystals commonly acute rhombs1 ,fig4 #fc4 ,twinning4--,occurs1 the twinning boundary bisecting the acute angles of the rhomb 7only noticed between crossed nicols71 ,fig4 #fd4 ,color4--,reddish-brown to yellowish to colorless4 ,illustration3 ,,fig4 #fc4--,titanite1 showing acute rhombic cross-section4 ,index of ,refraction4--n #a4ibj to #a4ifc 7 #a4hhh to #a4iac1 #a4igh to #b4jed71 hence relief very marked and surface very rough4 ,cleavage4--,imperfect and not parallel to predominant form1 hence only appears as a few rough cracks1 which are not parallel to any crystallographic boundary1 ,fig4 #fc4 ,cleavage rarely observed in secondary grains4 ,polarized ,light3 ,pleochroism4--,varies with the color1 being more distinct in colored crystals1 yellowish 7the lighter color7 a and reddish-brown c4 ,scarcely noticed when the color is light4 ,crossed ,nicols3 ,double ,refraction4--,very strong 7 #j4jij to #j4ada74 ,illustration3 ,,fig4 #fd4--,titanite1 showing twinning in nepheline-syenite4 7,from ,cohen47 ,interference ,colors4--,very high order1 like those of calcite4 ,due to the fact that the refractive indices of two of the rays are very nearly alike1 some sections may show very low order colors4 ,extinction4--,extinction angles not characteristic4 ,there may be no complete extinction in white light1 owing to dispersion4 ,convergent ,light3 ,on account of the very strong characteristic dispersion of the optic axes 7 71 the axial angle varies a good deal with the color of the light used1 7,st4 ,gotthard7 #b;,e,,ia #eg1 #b;,e,tl #dg4 ,by using colored glasses this variation in the axial angle can be seen4 ,the axial plane lies in the clino pinacoid 7#jaj71 hence bisects the obtuse angle in the rhombic cross-section1 ,fig4 #fc4 ,bxa47c7 #ea front4 ,the optical character is 774 ,alteration3 ,may take place4 ,distinguished from3 7a7 ,,staurolite4--,in convergent light the axial plane is shown to be in the shorter diagonal of the rhombic cross-section1 while in staurolite it is in the longer diagonal4 7b7 ,,rutile4--,by biaxial character4 7c7 ,,calcite4--,the light colored titanite 7sphene71 in absence of twinning1 by higher index of refraction4 ,titanite may easily be confused with some of the rarer minerals4 ,,remarks3 ,titanite is always an accessory mineral and is found distributed in all rocks1 except ,si,o#b rich eruptive and magnesia silicate rocks4 ,as a secondary mineral it forms rims around other titanium minerals or pseudomorphs after them and also the principal part of leucoxene4 ,it is partly soluble in hot hydrochloric acid and completely decomposed by sulphuric acid4 ,h41 #e to #e4e4 ,sp4 gr41 #c4c to #c4g4 ,in a specific gravity separation it falls with the ferruginous minerals 7on account of its density7 and from these can generally be separated by electromagnetic methods4 ,,feldspar ,,group4 ,orthoclase1 ,microcline and the ,plagioclases4 ,,orthoclase4 ,,anisotropic4 ,,biaxial4 ,,monoclinic4 ,,elongation 7 cleavage7 a4 ,,composition3 ,k,al,si#c,o#h1 with some replacement by ,na4 ,usual ,appearance in ,sections3 ,in crystals and grains4 ,in porphyritic rocks habit of crystals more or less tabular parallel to clino pinacoid 7#jaj71 or rectangular much extended parallel to clino axis 1 with cross-sections six-sided1 or rectangular to long lath shape1 ,figs4 #fe and #ff4 ,crystals may be changed into rounded or looped grains by chemical corrosion1 ,fig4 #f4 ,adularia crystals more prismatically developed1 giving rhombic sections4 ,dimensions of crystals vary extremely2 microlites occur1 at times forming sphrulitic structure4 ,the very fine grained ground-mass 8microfelsite0 7not resolved by the microscope7 consists largely of feldspar4 ,illustration3 ,orthoclase cleavage plates4 ,,fig4 #fe4--;,basal4 ,,fig4 #ff4--;,clino pinacoid4 ,intergrowths with microcline and plagioclase common1 forming 8microperthite0 when lamell are microscopic4 ,may be in zonal formation with plagioclase 7the orthoclase on the periphery74 ,also intergrown with quartz#ig forming 8pegmatite0 and 8micro-pegmatite10 ,fig4 #fg4 ,illustration3 ,,fig4 #fg4--,micro-pegmatitic structure1 in granophyric quartz-porphyry4 7,from ,cohen47 ,zonal structure often seen1 ,fig4 #bj1 especially when decomposition has commenced2 and in fresh crystals may be indicated by zonal arrangement of inclusions4 ,twinning4--,very common1 generally after ,carlsbad law1 ,figs4 #ah and #fi2 the twinning boundary1 dividing the section longitudinally1 either being parallel to edges of crystal or bent or jagged4 ,twinning after ,baveno 7twinning boundary diagonal1 with the two parts extinguishing at the same time1 but having a and c directions crossed in the two portions7 and ,manebach laws less common4#ih ,illustration3 ,,fig4 #fh4--,orthoclase1 ortho pinacoid section showing cleavages intersecting at #ij1 in augite-syenite4 ,color4--,colorless or tinged by oxide of iron4 ,cloudy if decomposed4 ,index of ,refraction4--n #a4ebc 7 #a4eai1 #a4ebf71 hence no relief and surface smooth4 ,cleavage4--,varies and sometimes only seen in very thin sections1 but is an important character and should always be searched for4 ,it occurs perfect1 parallel to base 7#jja71 and almost as perfect parallel to clino pinacoid 7#jaj74 ,the two cleavages intersect at #ij in section parallel to the b axis1 ,fig4 #fh4 ,inclusions4--,may be present and arranged in regular or zonal order1 but not important4 ,do not occur in individuals of a second generation4 ,polarized ,light3 ,pleochroism4--,none4 ,crossed ,nicols3 ,double ,refraction4--,very weak 7 #j4jjg74 ,interference ,colors4--,lower first order1 gray1 white1 etc41 not quite so bright as colors of quartz and plagioclase4 ,extinction4--,being monoclinic the extinction angle on base 7#jja71 with reference to clino pinacoid 7#jaj7 cleavage cracks1 is #j4 ,on clino pinacoid with reference to basal cleavage cracks1 it is #e4 ,some sections 7notably in glassy sanidine grains7 may appear dark during complete rotation4 ,this is due to the fact that the axial angle is very small and the sections act approximately like those of a uniaxial mineral at right angles to the optic axis4 ,convergent ,light3#ii ,plane of optic axes in general at right angles to clino pinacoid 7#jaj7 7plane of symmetry71 ,fig4 #fe1 hence parallel to trace of basal cleavage2 but in some sanidines parallel to plane of symmetry4 ,bxa 7a7 a #e above4 ,axial angles vary1 #b;,e #abe 7orthoclase71 #j-ej 7sanidine74#ajj ,may appear uniaxial when axial angle is very small4 ,optical character 774 ,alteration3 ,very common to clay1#aja muscovite1 hydrargillite1 etc4 ,generally commences along the cleavage cracks1 and when it has progressed very far the whole feldspar appears opaque or cloudy1 and no perceptible change may take place between crossed nicols4 ,as decomposition is very prevalent in many rocks1 the orthoclase is rarely clear or pellucid4 ,epidote is often formed when accessory solutions are present4 ,distinguished from3 7a7 ,the other ,,feldspars and ,,melilite4--,see under the latter minerals4 7b7 ,,quartz4--,feldspar is biaxial but when occurring in clear glassy grains 7notably sanidine71 which appear uniaxial in convergent light1 may resemble quartz4 ,when tested the optical character is 771 while that of quartz is 774 ,,remarks3 ,the members of the feldspar group are the widest distributed of the rock-forming minerals and their recognition is of the utmost importance on account of their bearing on the systematic classification of rocks4 ,orthoclase is found as an essential constituent in the more acid plutonic and older volcanic rocks1 as granite1 syenite1 trachyte1 porphyry1 and also in gneiss1 crystalline schists1 more seldom in contact rocks and subordinate in clastic rocks4 ,chemical corrosion 7producing rounded or looped grains71 ,fig4 #f1 and mechanical deformation 7producing angular1 sharp-edged1 broken grains1 bending and undulatory extinction#ajb71 ,fig4 #g1 occur in orthoclase4 ,when a rock containing feldspar crystals is shattered1 the orthoclase breaks parallel to basal cleavage and plagioclase parallel to twinning plane4 ,orthoclase is practically insoluble in acids4 ,h41 #f to #f4e4 ,sp4 gr41 #b4ef4 ,illustration3 ,,fig4 #fi4 --,sanidine1 showing ,carlsbad twin and cross-parting1 in nepheline-phonolite4 7,from ,cohen47 ,sanidine4--,this clear1 glassy variety of orthoclase occurs in the later eruptive rocks1 rhyolite1 trachyte1 obsidian1 etc4 ,sanidine often has a parting parallel to ortho pinacoid 7#ajj71 which may be noticed in sections so thick that the cleavage is not seen1 ,fig4 #fi4 ,in general it shows no sign of decomposition1 and has a smaller axial angle than orthoclase4 ,inclusions of glass are more abundant than in orthoclase4 ,,microcline4 ,,anisotropic4 ,,biaxial4 ,,triclinic4 ,,composition3 ,k,al,si#c,o#h4 ,usual ,appearance in ,sections3 ,as a rock constituent in irregular grains4 ,in general characters like orthoclase and distinguished from it and the plagioclases by characteristic 8gridiron0 structure between crossed nicols1 resulting from the polysynthetic twinning after both ,albite and ,pericline laws1 ,fig4 #ai4 ,this crossed twinning will show in all sections except those parallel to the brachy pinacoid 7#jaj74 ,the lamell are generally thinner than in the plagioclases and more 8spindle-shaped40#ajc ,furthermore the rather obscure triclinic crystallization is shown by an extinction angle of #ae on basal cleavage plates with reference to brachy pinacoid 7#jaj7 cleavage lines 7distinction from orthoclase1 which has ,o extinction angle74 ,,remarks3 ,found with orthoclase1 often almost replacing it1 in granite1 syenite1 gneiss1 etc41 and is one of the last minerals to form4 ,it is notably resistant to decomposition4 ,a structure like the microcline twin structure may be produced in orthoclase by dynamic action4#ajd ,,the ,,plagioclases4 ,albite1 ,oligoclase1 ,labradorite1 ,anorthite4 ,,anisotropic4 ,,biaxial4 ,,triclinic4 ,,elongation 7 ,albite twin lamell7 a 7except in anorthite when it may be a or c74 ,,composition3#aje ,albite1 ,na,al,si#c,o#h4 ,oligoclase1 n7,na,al,si#c,o#h7 ,ca,al#b,si#b,o#h1 or n ,ab ,an1 n #b to #f4 ,labradorite1 ,na,al,si#c,o#h n7,ca,al#b,si#b,o#h71 or ,ab n ,an1 n #a1 #b or #c4 ,anorthite1 ,ca,al#b,si#b,o#h4 ,usual ,appearance in ,sections3 ,much the same as orthoclase4 ,lath-shaped#ajf forms and microlites very common1 especially in the acid series4 ,illustration3 ,,fig4 #gj4--,plagioclase1 showing narrow lamell1 in diabase4 7,from ,cohen47 ,illustration3 ,,fig4 #ga4--,plagioclase1 showing broad lamell1 in gabbro4 7,from ,cohen47 ,twinning4--,polysynthetic1 after ,albite law1 almost universal2 the twinning appearing between crossed nicols as a series of dark and light bands1 bounded by parallel edges1 ,figs4 #gj and #ga4 ,the twin lamell are parallel to brachy pinacoid 7#jaj71 hence not observed in sections parallel to this pinacoid4 ,the lamell may appear irregular and interrupted1 and seem to be broader in the basic than in the acid series4 ,when this twinning fails1 however1 as in the basic plagioclases in certain metamorphic rocks1 the determination becomes very difficult4 ,in some cases polysynthetic twinning1 after both ,albite and ,pericline laws1 may take place at the same time1 giving rise to a structure somewhat similar to that of microcline1 ,fig4 #gb4 ,in addition the polysynthetic crystals may be twinned like orthoclase after ,carlsbad and ,baveno laws4 ,the general characters are the same as in orthoclase with the following differences3 ,indices of ,refraction3 n #a4ece 7 #a4ecb1 #a4edj7 ,albite4 n #a4eda 7 #a4ecg1 #a4ede7 ,oligoclase1 ,ab#d;,an#a4 n #a4eei 7 #a4eee4 #a4efc7 ,labradorite1 ,ab#a;,an#a4 n #a4ehb 7 #a4ege1 #a4ehh7 ,anorthite4 ,the surface of anorthite appears slightly rougher than that of orthoclase4 ,cleavages1 parallel to base 7#jja7 and brachy pinacoid 7#jaj71 never intersect at right angles1 as is the case in sections of orthoclase parallel to b axis4 ,this is due to the triclinic system of crystallization1 but the divergence from a right angle is small 7#ic #cf to #id #aj74 ,illustration3 ,,fig4 #gb4--,plagioclase1 showing crossed lamell1 in olivine-gabbro4 7,from ,cohen47 ,inclusions at times may be quite important1 as the vitreous inclusions of oligoclase in andesites1 etc41 and the iron ore inclusions and other microlites in labradorite4 ,the arrangement of these inclusions may be zonal or in parallel orientation4 ,double refraction is a little stronger than for orthoclase 7 #j4jjh to #j4jac 7anorthite771 hence producing slightly brighter interference colors in sections of the same thickness4 ,extinction takes place in all sections unsymmetrically with respect to crystallographic1 twinning or cleavage lines 7as these minerals are triclinic72 hence extinction angles are always observed4 ,convergent ,light3 ,all plagioclases show the emergence of a bisectrix1#ajg more or less oblique1 on brachy pinacoid 7#jaj7 cleavage faces4 ,these cleavage faces show no twin lamell1 unless twinning after ,pericline law occurs1 in which case the determination is much more complicated4 ,the axial angle is large1 #b;,e #aee 7,albite74 ,optical character1 depending on variety1 77 or 774 ,alteration3 ,partly the same as in orthoclase1 forming clay1 muscovite1 etc4 ,calcite and epidote are more common as side-products1 and zeolitization also occurs in some rocks4 ,the plagioclases decompose more easily than orthoclase4 ,distinguished from3 7a7 ,,orthoclase4--,by repeated twinning after ,albite law1 giving between crossed nicols a series of alternate dark and light bands4 ,when ,albite twinning is absent the distinction is very difficult4 7b7 ,,microcline4--,by common absence of the microcline 8gridiron0 structure between crossed nicols4 ,methods for ,optical ,determination of the ,plagioclases4#ajh ,the correct determination of the particular plagioclase is of the greatest importance in the classification of rocks1 and it is no longer sufficient to simply determine the feldspar as either orthoclase or plagioclase4 ,a quantitative analysis of isolated material would lead most surely to the desired result1 but has many objections4 ,modern optical methods now permit of a very accurate and convenient determination under ordinary circumstances4 ,but of course these methods involve a knowledge of the approximate orientation of the section tested4 ,when this section is not a definite cleavage fragment1 its orientation can best be determined by convergent light tests4 ,only an outline of these methods can be here given1 and reference should be made to more complete works for an elaborate discussion of the subject4 ,it is very convenient to have at hand a set of glass models of the plagioclases1 showing location of plane of optic axes1 vibration directions and crystal axes4#aji 7#a7 ,schuster's method of recognizing the different feldspars by extinction angles measured on the cleavage plates#aaj is very precise1 but not always applicable for crystals in rock sections4 ,,extinction ,,angles3 ,,extinction ,,angles3 ,,on ,,base1 ,,measured ,,from ,,trace ,,of ,,on ,,brachy ,,pinacoid1 ,,measured ,,from ,,pinacoidal ,,cleavage4 ,,trace ,,of ,,basal ,,cleavage4 ,albite #d ,albite #ai ,oligoclase1 ,ab#d;,an#a #b ,oligoclase1 ,ab#d;,an#a #h ,labradorite1 ,ab#a;,an#a #e ,labradorite1 ,ab#a;,an#a #bj ,anorthite #cf ,anorthite #da ,confusion may here arise between albite and labradorite if disregard be had to signs1 but the more acid oligoclase is readily distinguished from the basic anorthite4 ,by convention the angles on base and pinacoids are 77 when the direction of extinction has apparently moved as the hands of a watch1 with reference to the upper right hand edge 7between base and pinacoid7 of the crystal4 ,when the reverse is true the angles are 771 see ,fig4 #gc4 7#b7 ,the statistical method of ,michel ,lvy and others is often applicable1 especially in the following case3 ,sections at right angles to the brachy pinacoid 7#jaj7 and hence showing ,albite twinning4--,these sections1 as nearly perpendicular to the lamell as possible1 are known by the sharp dividing lines1 by the extinction angles on each side of the trace of the twinning plane being approximately equal and by the fact that the two adjacent lamell are of the same color when the trace of the twinning plane is parallel to the plane of vibration of either nicol4 ,also in the #de position the lamell are exactly the same color and the dividing lines disappear4 ,measure the extinction angles in as many sections thus selected as possible and take the maximum value4#aaa ,this should be very close to the maximum extinction angle1 which is a constant for each kind of feldspar4 ,illustration3 ,,fig4 #gc4--,showing conventional signs of extinction angles4 ,,maximum ,,extinction ,,angles ,,in ,,sections ,,perpendicular ,,to ,,albite ,,twins4 ,albite #af ,oligoclase1 ,ab#d;,an#a #e ,labradorite1 ,ab#a;,an#a #bg ,anorthite #ec ,in the determination of rod-like microlites1#aab oligoclase extinguishes almost parallel to its length1 while anorthite may show extinction angles of over #bg4 ,when these microlites show ,albite twinning use the method just described4 7#c7 ,fouqu's method#aac can be used when the optical orientation of the section is known 7as the result of a test with convergent light74 ,the extinction angles of these known sections are of great diagnostic importance4 ,the best sections are those at right angles to the two bisectrices1 and these may be obtained by rapidly testing those sections1 in the rock1 which show an interference color about half as high as the maximum color in the rock section1 in this way avoiding the sections parallel to the optic axes4 ,having found such a section1 test it with a gypsum or undulation mica plate to prove whether the bisectrix is a or c4 ,if a 7these sections show sharp twinning striations7 measure extinction angle between trace of axial plane and albite twinning2 if c measure extinction angle between trace of axial plane and basal cleavage cracks4 ,,extinction ,,angles ,,in ,,sections3#aad a1 ,,measured ,,from ,,albite ,,striations4 c1 ,,measured ,,from ,,cleavage ,,cracks4 ,albite #gd #ai ,oligoclase1 ,ab#d;,an#a #hh #e ,labradorite1 ,ab#a;,an#a #fj #bb ,anorthite #ee #dh ,when both extinction angles can be obtained1 the determination of the plagioclase is very certain1 but the result cannot be regarded as definite when only one is found2 and the method becomes more difficult as the crystals become smaller4 ,the position of the axial plane should be determined by convergent light test and not simply by the direction of extinction in parallel polarized light4 7#d7 ,michel ,lvy's method#aae can be employed when twinning is present after both ,carlsbad and ,albite laws4 ,illustration3 ,,fig4 #gd4--;,extinction ,angles in the ,zone normal to 7#jaj7 in ,carlsbad ,twins of the ,plagioclases4 ,the section to be tested should be in the zone perpendicular to 7#jaj74 ,such sections show sharp boundary lines between the twin lamell and1 between crossed nicols1 the ,albite lamell show the same interference color when the trace of 7#jaj7 is parallel to the cross-wires of the ocular1 and also in the #de position the lamell are the same color and no dividing lines show4 ,the two parts of the ,carlsbad twin exhibit different interference colors in the #de position1 this difference being more marked as the composition approaches that of ,anorthite4 ,the extinction angles are measured from the trace of the ,albite twinning plane 7#jaj71 paying regard to the and signs1 and the concurrent series of angles are to be obtained from the two parts of the ,carlsbad twin4 ,the range of these angles 7for four type compositions1 ,ab1 ,ab#d;,an#a1 ,ab#a;,an#a and ,an7 is given in the accompanying diagram for all positions of the section in the zone normal to 7#jaj74 ,in the curves of this diagram 7,fig4 #gd7 the vertical distances are the extinction angles for every ten degrees measured from the trace of the ,albite twinning plane1 and the horizontal distances represent varying positions of the section in the zone normal to 7#jaj7 for every ten degrees of rotation from the position to the edge 7#ajj77jaj7 7that is axis71 through a revolution of #ahj to again to the same position4 ,the concurrent angles in one part of a ,carlsbad twin are represented by a heavy line and in the other part by a broken line4 ,it will be observed that the difference between these concurrent angles is very small in ,albite 7#c7 and increases markedly towards ,anorthite 7#fj74 ,,table ,,for ,,becke ,,method4#aaf ,orthoclase ,quartz ,microcline ,albite ,oligoclase1 ,ab#d;,an#a 2 ,quartz4 ,labradorite1 ,ab#a;,an#a ,quartz ,anorthite 7#e7 ,becke's method may be employed to identify the feldspar1 by determining the relative values of the indices of refraction of the feldspar grain when it lies in contact with a quartz grain 7best results7 or with the balsam 7not such good results47 ,the grains should have vibration directions in parallel position4 ,other methods that may be employed are here simply referred to3 ,determination 7in convergent light7 of the emergence of an optic axis with reference to a known plane1 the basic plagioclases show an optic axis about parallel to of the crystal2 determination of total reflection by ,wallerant's total reflectometer2 determination of the value of the mean index of refraction of crushed isolated grains by ,schrder van der ,kolk's method2#aag determinations by specific gravity separations with use of heavy solutions1 and by chemical and micro-chemical tests 7for the relative amounts of ,k1 ,na and ,ca74#aah ,,remarks3 ,the plagioclases may have the same two general habits as orthoclase1 being glassy and colorless in the younger eruptive rocks1 and dull and cloudy in the granular and porphyritic1 older1 massive and schistose rocks4 ,they occur in rocks of intermediate and basic composition4 ,albite is found in granite 7commonly intergrown with orthoclase71 gneiss1 etc41 and frequently as a secondary constituent 7secondary feldspar#aai7 in the feldspar-quartz mosaic of mechanically metamorphosed rocks4 ,it may also be present in acid eruptive rocks4 ,oligoclase is very frequent in granite1 syenite1 gneiss1 diorite1 trachyte1 andesite1 diabase1 etc42 and particularly accompanies orthoclase4 ,labradorite is confined more to the gabbros1#abj basic eruptive rocks and crystalline schists1 rich in amphibole and pyroxene4 ,anorthite occurs in gabbros1 the most basic porphyrites1 basalts1 etc4 ,chemical corrosion and mechanical deformation#aba may take place as in orthoclase4 ,anorthite and labradorite are more or less decomposed by hydrochloric acid1 while albite and oligoclase are not acted on by the acid4 ,especially interesting is the alteration of the plagioclase that takes place in gabbros1 accompanied by 8uralitization0 of the pyroxene1 forming 8saussurite40 ,this consists of a white to greenish confused aggregate1 chiefly of zoisite1 grossularite1 vesuvianite1 chlorite1 secondary feldspar 7albite71 etc4 ,anorthoclase 7a ,na ,k1 triclinic1 feldspar74--,shows between crossed nicols intersecting areas of exceedingly fine composite twin structure and others of homogeneous structure1 producing a watery or 8moir0 appearance4 ,the twin structure may be only seen in very thin sections4 ,all possible kinds of perthitic intergrowth occur4 ,further distinguished from orthoclase by small extinction angle 7#d7 on base and by smaller axial angle 7#b;,e #gb to #hh74 ,replaces orthoclase in the ,na rich eruptives4 ,found in augite-syenite and 8,rhombenporphyr0 of ,norway 7with rhombic cross-section71 acid augite-andesite of ,pantelleria and in the porphyries of the ,hartz4 ,,cyanite1 ,disthene4 ,,anisotropic4 ,,biaxial4 ,,triclinic4 ,,composition3 ,al#b,si,o#e4 ,,elongation c4 ,illustration3 ,,fig4 #ge4--;,cyanite1 macro pinacoid cleavage section4 ,usual ,appearance in ,sections3 ,blade-like crystals without terminal planes1 but with cross-section 7six-sided7 showing two long parallel edges and four shorter edges2 also in columnar aggregates4 ,twinning common1 with generally twinning plane parallel to macro pinacoid 7#ajj74 ,colorless or bluish and spotted4 ,the index of refraction is high 7n #a4gbj1 #a4gab1 #a4gbh71 hence relief marked and surface rough4 ,cleavage perfect1 parallel to macro pinacoid 7#ajj71 appearing as sharp cracks1 parallel to longest edges in cross-sections2 less distinct1 parallel to brachy pinacoid 7#jaj74 ,fibrous parting parallel to base 7#jja71 ,fig4 #ge4 ,pleochroism 7colorless to blue c7 not noticed except in colored crystals4 ,crossed ,nicols3 ,double refraction quite strong 7 #j4jaf74 ,interference colors upper first order1 yellow1 red1 violet1 etc4 ,extinction angles observed in all sections 7being triclinic71 reaching a maximum of #cj on macro pinacoid 7#ajj71 ,fig4 #ge4 ,extinction on base1 about parallel to most perfect cleavage4 ,in convergent light axial angle large2 axial plane and ,bxa4 about perpendicular to best cleavage 7#ajj72 optical character 774 ,alteration3 ,seldom observed1 but may take place to mica4 ,distinguished from3 7a7 ,,amphibole by cleavage 7intersecting cleavages at #abd in amphibole and #ij in cyanite7 and by 7#ajj7 cleavage plates of cyanite showing emergence of acute bisectrix4 7b7 ,,corundum by being biaxial4 ,distinction from similar appearing minerals may be difficult4 ,,remarks3 ,found in gneiss1 granulite1 metamorphic schists1 ecolgite1 etc41 commonly associated with garnet4 ,it is not attacked by acids4 ,h41 #e to #g4 ,sp4 gr41 #c4f4 ,,serpentine4 ,,aggregate4 ,,elongation 7of fibers7 c4 ,,composition3 ,h#d,mg#c,si#b,o#i1 with replacement by ,fe4 ,usual ,appearance in ,sections3 ,dense1 fibrous 7chrysotile7 or scaly 7antigorite7 aggregates4 ,color4--,colorless to light greenish1 except the ,fe rich variety which is green4 ,index of ,refraction4--n #a4ee to #a4ef 7 #a4ef1 #a4ega for antigorite71 hence no relief and surface smooth4 ,polarized ,light3 ,pleochroism4--,not seen or very feeble1 except in the ,fe rich variety4 ,crossed ,nicols3 ,double ,refraction4--,rather weak 7 #j4jji to #j4jaa74 ,interference ,colors4--,middle first order1 gray1 white1 yellow1 etc4 ,anomalous colors do not appear4 ,the aggregate structure is distinctly seen between crossed nicols4 ,due to compensation aggregates may appear isotropic4 ,distinguished from3 ,,chlorite4--,by more usual absence of color1 pleochroism and anomalous interference colors2 but this distinction may be very difficult4 ,,remarks3 ,serpentine 7both antigorite and chrysotile7 is essentially a secondary mineral1 resulting in most cases from the alteration of chrysolite 7olivine71 ,fig4 #bb1 more rarely of pyroxene or amphibole4#abb ,the alteration of olivine to antigorite leads to the characteristic 8lattice structure10 the alteration to chrysotile to 8mesh structure40 ,in the case of the 8mesh0 formation the alteration starts from the surface and cracks1 producing fibres of chrysotile1 which stand at right angles to these edges and cracks4 ,as serpentinization proceeds new cracks form1 due to increase in volume1 and the process may continue until complete pseudomorphism takes place4 ,when this subsequent serpentinization of the meshes takes place the resulting serpentine may appear almost isotropic#abc and is certainly different from the chrysotile of the first formed veins 7,weinschenk74 ,pieces of the parent mineral are often present4 ,serpentine is found in ophiolites1 the altered basic igneous rocks1 pyroxenites1 peridotites1 etc41 and as a primary mineral in the ,central ,alps peridotite1 intergrown with fresh olivine 7,weinschenk74 ,it may also form a rock by itself4 ,serpentine is attacked quite strongly by hydrochloric acid1 still more so by sulphuric acid4 ,common serpentine is not altered by heating 7distinction from chlorite71 but the ,fe rich variety becomes brown and opaque4 ,h41 #b4e to #d4 ,sp4 gr41 #b4e to #b4g4 ,,clay1 ,kaolin4 ,,composition3 ,,aggregate4 ,h#d;,al#b,si#b,o#i 7kaolinite74 ,usual ,appearance in ,sections3 ,fine1 scaly1 colorless aggregates1 which appear opaque 7due to porous structure74 ,the scales show basal cleavage4 ,index of refraction is about the same as balsam 7n #a4ee71 hence no relief4 ,the double refraction is weak 7 #j4jjh74 ,distinguished from3 ,colorless ,,mica and ,,hydrargillite 7,al7,,oh7#c71 which as an alteration product of the feldspars is often confused with clay by weak double refraction4 ,,remarks3 ,clay results from the alteration of the feldspars 7especially the plagioclases71 elolite1 scapolite and other silicates4 ,kaolinite is insoluble in hydrochloric but decomposed by sulphuric acid4 ,h41 #b4e4 ,sp4 gr41 #b4f4 ,,chapter ,v4 ,,methods ,,of ,,preparing ,,sections4#abd ,the methods described will be those used in making rock sections4 ,the general principles involved are the same for both rock and crystal sections2 although more difficulties are encountered in making the latter1 due to the sections being required in some definite direction or of some precise thickness1 or on account of the possibly fragile or brittle character of the crystals4 ,in the case of rock sections any section1 taken 8at will0 through the rock1 will generally do4 ,cutting and ,grinding ,machines4#abe ,many different kinds are used1 the power being furnished by steam1 electric motor#abf or by the hand or foot4 ,the chip or fragment of rock to be sliced should be comparatively small in size1 and may be cut by either a vertical diamond saw or by a horizontal emery-disc4 ,the fragment is held in position by suitable guides4 ,the moistened emery can be kept in the tray and fed to the disc by a spoon or some other device4 ,diamond saws are only needed in the case of hard rocks4 ,for cutting sections of crystals in definite directions a form of small hand apparatus#abg is shown in ,fig4 #gf4 ,this cut shows how the plate1 to which the crystal is cemented1 can be given definite rotation in two directions4 ,the simpler crystal holder1 appearing loose on the stand1 may be used when a section is required parallel to the face of the crystal cemented to the holder4 ,in these machines the cutting is done with rotating saws of sheet-tin1 usually charged with either diamond dust or emery4 ,illustration3 ,,fig4 #gf4 ,in the case of a very small crystal a section1 parallel to any desired face1 can be obtained by cementing this face to a suitable frame or holder and grinding down by hand on a glass plate with emery1 the final polish being given with rouge4 ,when the section required is not parallel to a crystallographic face or cleavage it must be verified geometrically with reference to other faces4 ,if the crystals are soluble in water1 some other liquid1 as bromnaphthalin or benzine1 must be used in grinding2 and very fragile crystals are rubbed down on a ground glass plate without emery or rouge1 simply using bromnaphthalin or some other appropriate liquid4 ,when sections of a crystal are desired with strictly parallel faces1 the form of grinding apparatus shown in ,fig4 #gg can be used4 ,this apparatus consists of a cylinder1 held within a suitable frame supported on three set-screws1 a#a1 a#b and a#c1 of hard steel4 ,the lower surface of the frame and consequently the bottom of the cylinder can be adjusted by the wedge k and the set-screws so that it is exactly parallel to the grinding surface of the glass plate4 ,the crystal to be ground down1 p1 is cemented to the bottom of the cylinder1 and the whole apparatus rubbed over the grinding surface of the plate4 ,the pressure of the hand on the upper part of the cylinder regulates the pressure of the crystal on the grinding surface4 ,in this way a surface is obtained which is exactly parallel to the surface cemented to the cylinder4 ,illustration3 ,,fig4 #gg4 ,sections of definite thickness can also be obtained by using this apparatus2 for the adjustments with the wedge k may be so arranged that1 when the stop t in the cylinder has reached the bottom of the slot in the frame1 the lower surface of the cylinder will be the required distance from the grinding surface of the glass plate4 ,the wedge is graduated so that the value of one division on the sloping top is known in mm4 of vertical distance4 ,saws4 ,saws1#abh of about six inches in diameter made from inch sheet-tin1 are convenient for general work1 and may be charged with diamond dust by the operator as follows3#abi ,crush one or two carats of rough crystals of diamond-bort in a small steel mortar to about the condition of fine sand1 care being taken not to reduce the diamond to powder4 ,transfer the dust to a piece of flat iron and1 after collecting it in a heap1 moisten with a drop of oil4 ,the cutting edge of the tin disk must now be prepared by making a series of incisions 7 inch deep7 on the outer margin4 ,this can be done by striking the disk with a sharp1 thin knife edge4 ,the larger the number of incisions and the closer they are together the better4 ,charging the saw is accomplished by gently hammering the edge against the iron plate1 upon which is the paste of diamond dust and oil4 ,the disk must be slowly rotated during this charging process1 which should be continued until all portions of the edge have been gone over two or three times4 ,instead of charging with diamond dust the saws may be used with emery4 ,the edges of the saws should first be 8upset10 as just described1 and then while rotating charged with emery4 ,the emery in the state of mud can be applied with the thumb and fingers1 or the saw can be allowed to pass through a tray of emery mud4 ,the finer grades of emery 7,nos4 #ajj to #abj7 should be used4 ,the method of mounting saws1 of course1 depends on the way the saws and spindles are constructed4 ,it is well1 however1 to have the hole in the center of the saw-disk sufficiently large to allow of adjustment for accurate centering4 ,cutting4 ,during the process of cutting a lubricant must be used on the saw4 ,petroleum has been recommended by some1 but water seems to be the most used4 ,holding a piece of soap against the cutting edge will also reduce friction4 ,it is convenient to carry the water to the saw through a very small lead pipe1 which permits of easy adjustment1 so that the water can be delivered at just the right spot1 the flow of water1 of course1 being regulated by a stop-cock4 ,another method for cutting sections is used by the ,u4 ,s4 ,geological ,survey at ,washington1 ,d4 ,c4 ,the apparatus consists of an endless wire of soft iron1 about inch in diameter1 revolving over two pulleys or wheels1 one of which is driven by steam-power4 ,the wire is charged by dipping the fingers in water1 taking up a little fine 7#abj grade7 emery and holding the fingers against the wire above the section4 ,the wire revolves so as to cut downwards1 thus carrying with it the emery taken from the fingers4 ,in cutting sections the thickness1 of course1 depends on the character of the rock2 the more porous and fragile the rock the thicker the section should be made1 an ordinary section being about as thick as a silver quarter of a dollar4 ,very often by skill and practice a suitably shaped chip or fragment can be detached from the specimen with a hammer or with a hammer and cleavage chisels1 thus avoiding the delay and trouble involved in the use of the section cutter4 ,grinding ,plates or ,laps4 ,copper laps1 about #g-h inches in diameter and inch thick1 seem to be best for general purposes2 although laps made of lead or cast iron are also used4 ,a lead lap1 being soft1 holds the emery well and thus hastens the grinding away of the section2 but at the same time its own smooth surface is soon destroyed4 ,a cast iron lap is much harder and retains its smooth surface better1 but does not hold the emery so well and therefore retards the process of grinding4 ,copper1 being intermediate in hardness between lead and iron1 seems to combine the advantages of both4 ,it is always good practice to have at least two laps1 one for the preliminary grinding with coarse emery and the other for the final grinding with the finest grades of emery4 ,in this way one lap is always kept with a smooth surface1 making it possible to give a uniform1 plane finish to the section4 ,the surface of the laps should not be exactly plane1 but turned or finished so as to be a little higher 7 of an inch for diameter of #h inches7 at the axis than at the periphery2 the idea being to compensate for the more rapid abrasive action towards the periphery4 ,cementing4 ,for grinding down to a proper thinness for transparency it is necessary to cement the fragment of rock to a holder1 which consists of a small piece of plate-glass about inch thick4 ,if the piece of rock has been cut by a good diamond saw the surface will be comparatively smooth and uniform1 and may only need polishing 7described under grinding7 before cementing4 ,when1 however1 the piece of rock is in the shape of a chip or rough fragment it is first necessary to prepare a smooth ground surface on one side 7described under grinding7 before it can be cemented to the glass holder4 ,the pieces of plate-glass used should be free from flaws1 bubbles or anything tending to destroy uniformity of surface1 and should be a little larger than the chips and slices of rock for which they are to act as holders4 ,hardened ,canada balsam1 or better yet1 a cement made of a mixture of shellac and ,venice turpentine1#acj should be used4 ,the process of cementing is carried on as follows3 ,heat the glass support over an alcohol lamp or a ,bunsen burner1 place it on a good non-conductor of heat1 as a piece of wood1 and rub it with a stick of the cement until a sufficient quantity has been melted off4 ,the piece of rock should then be treated in the same manner1 applying the cement to the smooth finished portion4 ,care must be taken that neither the glass nor rock preparation are heated too much1 as this would cause the cement to smoke or bubble4 ,after placing the rock preparation and plate together1 use quite a little pressure in order to drive out all superfluous cement4 ,examine now1 through the glass1 the contact surface and see that it is entirely free from bubbles1 etc4 ,bubbles are usually caused by overheating of the parts before cementing4 ,if only very few bubbles are seen1 they may generally be removed by moving the rock preparation rapidly back and forth over the glass plate4 ,if the bubbles are numerous or cannot be removed in the manner just described1 the rock preparation must be separated from the glass plate and the cementing done over again4 ,bubbles must not be allowed to remain1 as the portions of the rock over them1 not having any cement backing1 are ground away leaving holes in the section4 ,grinding4 ,after the rock preparation has been cemented to its glass holder it is ready for the grinding process1 for which are used horizontal laps1 mounted on a vertical spindle in the tray ,t1 ,fig4 #gf4 ,the copper lap for the first grinding is put in place1 and1 after it has commenced to rotate1 is charged with emery and water4 ,this can conveniently be done by dropping on the emery and water1 either with the fingers or with a small bunch of rags at the end of a stick4 ,a large camel's hair brush may be used for putting on the finest emery4 ,the emery is distributed over the rotating lap by the motion of the rock preparation during grinding4 ,the kind of emery used for this first grinding depends somewhat on the character of the rock section4 ,if the section is composed of a hard1 compact1 fine grained rock1 the first emery used may be quite coarse2 the coarser the emery the more rapid being the grinding down4 ,for rough1 quick work1 ,nos4 #hj to #ajj emery can be used until the section becomes quite thin2 then1 still using the same lap1 substitute the finest corn emery 7next coarsest to flour emery7 and continue grinding until the section becomes quite translucent4 ,the lap must then be changed1 and the grinding finished with the finest emery dust4 ,too great care cannot be given to cleanliness while doing this sort of work4 ,a single grain of coarse emery1 getting on the lap or section during the final grinding1 will often spoil the whole work by making a bad scratch4 ,in grinding down the section1 great care must be taken to see that the grinding surface is kept parallel to the surface cemented to the glass4 ,the section is firmly pressed against the lap by means of two or three fingers1 depending on its size1 and uniform pressure should be maintained over all parts4 ,the section should be examined frequently1 and if it is noticed that one part is thicker than another1 place the section on the lap so that the thicker part is towards the periphery1 and put a little more pressure upon this part4 ,gutting or grooving the copper laps is prevented by moving the section from center to periphery and back1 at the same time giving to it a slight rotary motion4 ,it is best to have the lap revolve from right to left 7in the opposite way from the hands of a watch71 as1 holding the section on the right side of the lap1 it is easier to keep it in place by a pulling motion rather than by pushing it against the motion of the lap4 ,in the final stages of grinding and polishing great precaution must be exercised1 as one or two turns too many of the lap will often tear away a large part of the very thin section4 ,the section should be frequently examined1 by transmitted light1 with a small1 low power microscope1 a drop of water or oil being put on the section to render it more transparent4 ,if the section is thick and fastened to the plate-glass with a good deal of cement1 it will be found convenient1 after the section has been ground quite thin1 to remove most of this cement from around its edges4 ,if this is not done1 it may delay the final work of grinding down4 ,a good rock section should be made so thin that all the transparent and translucent component minerals can be examined and studied4 ,well made rock sections average in thickness from #j4jc to #j4je mm4 ,the last part of the final grinding and polishing can be done by the hand on a ground glass plate1 using the finest emery dust or rouge and water4 ,flour emery may also be used entirely for grinding1 and the finest emery dust for polishing4 ,the finer emery makes better work1 although it takes more time1 and is much safer for porous or brittle sections1 as the danger of tearing away pieces of the section is avoided4 ,in the case of a porous or decomposed rock it is well before grinding to boil the section in ,canada balsam or other equivalent material so as to fill all the pores and interstices1 thus making the section more compact and less liable to chip away4 ,sometimes it may even be best before grinding to mount the section on its permanent glass slide1 so that when it has been ground thin enough the cover can be put on without running any risk in transferring the section4 ,the only objection is that the glass slide may be a little scratched by the emery4 ,mounting4 ,when the section is sufficiently thin and has a smooth uniform surface it is ready to be transferred and mounted upon its permanent glass slide4 ,the glass plate1 which has held the section during the grinding1 is gently heated1 and1 as soon as the cement begins to soften1 the section is very gently pushed off into a shallow cup or evaporating dish containing turpentine or alcohol4 ,turpentine is convenient for general use1 because common alcohol 7on account of the water it contains7 unites with the shellac to form a kind of white pasty substance1 which is sometimes hard to remove from the section4 ,the section should be carefully cleaned in the turpentine or alcohol bath by means of fine camel's hair brushes4 ,a section-lifter1 made from a piece of broad1 flat watch-spring fastened to a small handle1 is then gently placed under the section1 which is lifted out of the bath in an inclined position so as to allow the liquid to drain off4 ,if a drop still adheres it can be removed by touching it gently with the finger4 ,the section is now placed in a tray1 upon a piece of white1 unglazed paper1 so that it may thoroughly dry2 after which process it is transferred to a glass slide1 and1 if necessary1 its rough edges chipped off with a knife4 ,if the section is very large1 and it is desired to mount it in two pieces1 it can easily be divided by holding one half tightly between two glass slides and gently bending the other half with the fingers4 ,the fracture will take place along the edges of the glass slides4 ,a mixture of gum damar and benzole is recommended for mounting1 the claim being made that gum damar does not turn yellow with age1 whereas ,canada balsam may do so4 ,two solutions of gum damar and benzole are used4 ,one very thin1 about the consistency of water1 the other quite thick1 about the consistency of mucilage4 ,both solutions are prepared by dissolving the gum damar in benzole and filtering through a linen or silk rag4 ,if the solution is too thin it can be thickened by placing the bottle in a warm place and allowing the excess of benzole to evaporate1 or if too thick1 more benzole can be added4 ,it is convenient to keep the solutions in short glass bottles1 with wide necks and glass covers instead of corks4 ,the solutions can be applied with glass rods4 ,when the section is all ready for its final mounting it is carefully centered with a coarse needle on the glass slide1#aca and a drop of the thin solution placed at the edge of the section4 ,capillary attraction will cause it to flow over the glass slide under the section1 and while it is soft the section can be again adjusted in place4 ,the slide is then placed in a cool1 dust proof place and allowed to stand for about twelve hours1 when the cement will be set and the section held firmly in place4 ,the upper surface of the section is then washed with a drop of benzine1 and as much as may be necessary of the thick mounting solution placed upon it with a glass rod4 ,a cover of glass of the right size is gently heated and placed in position over the section4 ,care must be taken not to have the cover too hot1 as it would cause bubbles to form in the cement4 ,it is also best to rest one edge of the cover on the cement first1 and then lower it gently so as to prevent air bubbles from being included4 ,the cover is adjusted and then held firmly in position by means of a mounting clamp1 which presses out all superfluous cement4 ,if a few small bubbles remain near the edges they may be let alone1 as they will generally work out by themselves in time4 ,sometimes a large bubble can be worked out by local pressure with the finger and gentle heating with a small iron rod4 ,if the mounting is unsatisfactory the cover should be removed by heating1 the old cement washed off with a camel's hair brush and benzole1 and a fresh cover put on4 ,after the cement has set1 the clamp may be removed1 but the slide should be left for #bd hours1 or longer if necessary1 until the cement is quite hard4 ,this takes longer in hot than in cold weather4 ,the advantage of using two solutions for mounting is that the section is firmly held in place on the glass slide while the cover is being placed over it2 there is thus no slipping or sliding of the section1 and when the cover is adjusted in place the work is well finished and the section still in the centre of the slide4 ,canada balsam1 or balsam in xylol1 are also often used for mounting4 ,the advantage of the latter being its quick drying or 8setting40 ,cleaning and ,finishing4 ,when the cement has set the superfluous part is singed by means of a small hot iron rod4 ,care must be taken not to use too large a rod or to have it too hot1 as the cement under the cover might then soften and allow the cover to slip or air bubbles to form4 ,the singeing drives off the more volatile part of the cement1 leaving only a brittle residue1 which can be easily scraped off with a knife4 ,in some cases it may be necessary to singe and scrape twice4 ,the final cleaning is done with a soft rag and benzole4 ,the slides are now ready to be marked1 either with a diamond pencil or with pasted labels4 ,convenient ,apparatus for ,work in a ,petrographical ,laboratory3 ,section lifters made of watch springs1 three sizes4 ,section holders or clamps for pressing out superfluous cement4 ,needle points mounted in light handles4 ,easy spring forceps4 ,mounting frame for centering section on glass slide4 ,small iron rod for singeing4 ,small camel's hair brushes4 ,cutting pliers and forceps4 ,set of wooden section trays4 ,glass stopper bottles for benzole1 benzine1 turpentine and alcohol4 ,open neck bottles1 with covers for two mounting liquids4 ,rotating mounting stand4 ,small microscope for testing transparency of section4 ,glass rods4 ,small squares and rectangles of plate-glass4 ,round1 oval and square cover-glasses4 ,glass slides with ground edges4 ,,chapter ,,vi4 ,,chemical ,,and ,,mechanical ,,tests4 ,these tests may be necessary to confirm an optical determination or to assist in the differentiation of the closely related species of a group1 as for example1 the different plagioclases in the feldspar group4 ,they may also be useful in the case of opaque substances4 ,the ordinary chemical methods employed in mineral analyses are often not applicable1 on account of the minute size of the mineral under investigation and the lack of sharpness in the reactions4 ,those methods#acb are to be preferred which produce crystallizations independent of the relative proportions of the materials taking part in the reaction and also of the physical conditions invoked4 ,the tests can be made either on the crystal in a rock section or on the isolated crystal or fragment1 the latter method being preferable when possible4 ,,chemical ,,tests ,,made ,,on ,,crystal ,,in ,,section4 ,the part of the section to be tested must be prepared by thorough washing with alcohol and benzole to remove all traces of balsam4 ,if the section is covered1 the cover-glass can be cut across with a diamond1 and1 after heating1 the desired portion removed with a knife edge4 ,any portion of a section can be isolated by surrounding it with a rim of viscous balsam or by putting on a new cover-glass1 in which a small hole has been made4#acc ,this hole can be accurately adjusted over the special portion of the section and the balsam removed by alcohol and benzole4 ,the treatment of rock sections with ordinary acids1 such as hydrochloric1 may show the presence of easily soluble minerals#acd and carbonates1 or distinguish silicates that are soluble with jelly1 or produce etched figures on the minerals4 ,test for ,carbonates4 ,when only present in very minute grains the test can be made as follows3 ,cover the rock section with a drop of water and a cover-glass1 then allow a drop of acid to slowly diffuse through the water film4 ,the glass cover will prevent the escape of the gas bubbles#ace which will thus surely be detected4 ,if necessary the section can be warmed by heating the projecting part of a suitable stand1 placed under the section on the stage of the microscope4 ,test for ,gelatinizing ,silica4 ,cover the carefully cleansed section with a little dilute acid 7commonly hydrochloric7 and let stand4 ,if too much acid is used the resultant gelatin will spread over the whole section and not appear simply on the gelatinizing silicate4 ,warm the section1 if necessary1 and finally rinse off the remaining acid thoroughly with water4 ,do not allow the action of the acid to continue too long1 as it is desirable to obtain only a very thin film of gelatinous silica over the minerals attacked1 so that the optical tests can still be made on these minerals4 ,if after the first trial the action has not been pronounced enough1 the test should be repeated4 ,the transparent film of gelatinous silica is made more visible by covering the section with a drop of water1 containing a dilute solution of fuchsine4 ,after standing for some time the section should be washed1 when only those portions covered by the gelatinous film will show the color stain4 ,,etched ,,figures4#acf ,the results of etching tests can not be regarded as very satisfactory in the case of sections of minerals in rock sections1 on account of doubt as to the crystallographic orientation of these sections4 ,the symmetry of the etched figures depends essentially on the relation of the crystal faces on which they are obtained to the planes of symmetry4 ,the forms of the etched figures differ on the same face of a mineral1 depending on the reagent used1 but their degree of symmetry is independent of the reagent or its degree of concentration4 ,the sharpest figures are produced on crystal and cleavage faces1 the figures being less perfect on artificially prepared faces1 even when polished4 ,the etching tests may1 however1 be tried to prove the presence of twinning1 or to distinguish between minerals of similar appearance but belonging to different systems4#acg ,various acids or alkalies are used to produce the etched figures1 depending on the mineral to be tested4 ,different factors influence the formation of good etched figures1 and that method must be used which seems to give most satisfactory results in the given case4 ,the action of the reagent should be sufficiently pronounced to develop clearly the etched figures2 at the same time the tests must be stopped before the solvent action has been too powerful4#ach ,after treatment for etching the section should be thoroughly washed and examined in some fluid of weak refraction 7with n lower than that of the crystal section71 such as water or air4#aci ,the objective1 of course1 must be focused on the surface of the section4 ,,heating ,,sections ,,to ,,redness4#adj ,the part of the section to be tested must be removed from the object glass1 carefully cleansed of balsam1 and held on platinum foil in the oxidizing blowpipe flame4 ,after the test the fragment used may be remounted in ,canada balsam for study4 ,as a result of heating3 ,colorless1 hydrous minerals 7zeolites and chlorites7 become cloudy in appearance4 ,colorless silicates1 containing protoxide of iron 7as olivine1 or faintly colored pyroxene or amphibole71 become red or reddish-brown4 ,colored minerals may change their color1 chloritic substances becoming brown or black if heated enough4 ,hornblende always becomes pleochroic1 and olivine sometimes becomes so4 ,members of the sodalite group may be turned blue1 if not already of that color4 ,the dichroism 7yellow to blue7 of almost colorless iolite may be developed4 ,carbonaceous particles may be distinguished from the iron oxides by being consumed4#ada ,,methods ,,of ,,isolating ,,crystals ,,or ,,mineral ,,fragments ,,for ,,testing4#adb ,for the application of these methods the rock or aggregate of minerals should be reduced to homogeneous#adc grains of uniform size 7preferably crystals or cleavages7 and not to powder4 ,this is best done by pounding in a metal mortar1 avoiding all grinding motion4 ,the separations required may be made by specific gravity solutions or magnetic methods1 alone or combined2 and in some cases may be assisted by chemical action4 ,when other methods fail it may be necessary to separate single grains from a mixture by hand4 ,a grooved piece of plate-glass1 passed beneath the objective of the microscope1 will be found useful4 ,the desired grains in this groove can be picked out by means of a piece of fine waxed thread or a fine pointed stick moistened at the end4 ,the hardness of the homogeneous grains may be obtained by pressing them firmly into the end of a lead stamp or holder and trying the effect of scratching upon the faces of minerals of known hardness4 ,specific ,gravity ,separation4#add ,accomplished by use of fluids of different specific gravity4 ,these fluids can be made specifically lighter by dilution1 and hence the fragments will fall to the bottom in order of decreasing density4 ,dilution of the heavy solutions to any specific gravity may be affected empirically until the solution will just suspend a fragment of a mineral having the desired specific gravity2 or the exact specific gravity of the solution may be determined by the ,westphal balance4 ,the following indicators may be employed to determine the limits of the specific gravity of the solution to be used for separation purposes 7,v4 ,goldschmidt73 ,,no4 ,,name4 ,,locality4 ,,sp4 ,,gr4 #a4 ,sulphur1 ,girgenti1 #b4jgj #b4 ,hyalite1 ,waltsch1 #b4afj #c4 ,opal1 ,scheiba1 #b4bab #d4 ,natrolite1 ,brevig1 #b4bdf #e4 ,pitchstone1 ,meissen1 #b4bhd #f4 ,obsidian1 ,lipari1 #b4cfb #g4 ,pearlite1 ,hungary1 #b4cig #h4 ,leucite1 ,vesuvius1 #b4dfe #i4 ,adularia1 ,st4 ,gotthard1 #b4egj #aj4 ,elolite1 ,brevig1 #b4fag #aa4 ,quartz1 ,middleville1 #b4fej #ab4 ,labradorite1 ,labrador1 #b4fhi #ac4 ,calcite1 ,rabenstein1 #b4gae #ad4 ,dolomite1 ,muhrwinkel1 #b4gcc #ae4 ,dolomite1 ,rauris1 #b4hfh #af4 ,prehnite1 ,kilpatrick1 #b4iaf #ag4 ,aragonite1 ,bilin1 #b4icc #ah4 ,actinolite1 ,zillerthal1 #c4jbj #ai4 ,andalusite1 ,bodenmais1 #c4abe #bj4 ,apatite1 ,ehrenfriedersdorf1 #c4ahj ,among the heavy solutions employed may be mentioned3 ,thoulet's solution of potassium-mercuric iodide 7,,ki3 ,hg,i#b #a3 #a4bd71 maximum specific gravity #c4aif4 ,klein's solution of cadmium borotungstate 7#b;,h#b,o1 #b;,cd,o1 ,b#b,o#c1 #i,wo,o#c #af;,h#b,o71 maximum specific gravity #c4f4 ,these two solutions can be mixed with water in any proportion without being decomposed4 ,solution of barium mercuric iodide1 maximum specific gravity1 #c4ehh1 cannot be diluted with water4 ,methylene iodide 7,,ch#b;,i#b71 specific gravity #c4cbdc at #af ,c4 varying with the temperature1 can be diluted with benzole but not with water4 ,nitrates of silver and thallium#ade 7,ag,n,o#c3 ,tl,n,o#c #a4a7 fuse at about #ge ,c4 to a clear mobile liquid with specific gravity over #d4e4 ,can be mixed while melted with water in all proportions2 but cannot be used for separation of sulphides1 as these nitrates are attacked by them4 ,possible chemical action between the minerals and heavy solutions must not be overlooked in this method of separation4 ,the funnel-shaped apparatus for these separations must be so arranged1 with stop-cocks1 etc41 that the heavier material collected at the bottom can be easily drawn off or removed at any stage of dilution4 ,these separations1 for various reasons1 are not always complete1 but the best results are obtained when the processes are repeated several times4 ,electro-magnetic ,separation4 ,all iron-bearing minerals may be separated from those free from iron by an electro-magnet4 ,the factors influencing the attraction of a mineral by an electro-magnet are not definitely known1 and do not seem to depend only on the percentage of iron4 ,minerals1 such as amphibole1 pyroxene1 epidote1 olivine and garnet 7containing iron71 may often be separated by an electro-magnet by regulating its magnetic intensity4#adf ,separation by ,chemical ,means4 ,very many different methods may be used1 depending on the nature of the work to be accomplished2 but they are generally only reliable in the hands of a good chemist4 ,the material should be in the state of fine powder4 ,as an example may be mentioned the treatment with pure concentrated ,h,fl1 by which the minerals of a rock are attacked in a certain sequence1 the feldspars and related minerals first1 then the quartz and finally the ferro-magnesium silicates1 such as amphibole1 pyroxene1 olivine1 etc4 ,,micro-,,chemical ,,reactions4 ,the first requisite is to bring the substance to be investigated into solution4 ,this can be done in the case of non-silicates by the ordinary solvents1 while silicates can be decomposed and investigated either by the methods of ,borichy or ,behrens4 ,both methods rest upon the recognition of the forms1 etc41 of artificially produced crystals4 ,the size of a fragment for testing may vary1 according to circumstances1 from that of a poppy seed to a pin head4 ,good results are recorded from fragments of not more than #j4b-j4g sq4 mm4 ,the substance to be tested is placed on glass1 protected by a film of balsam1 and covered with a spherical drop of the solvent1#adg which should be allowed to act until all the different elements composing the sample are in solution 7in the case of a very small fragment until it has all dissolved74 ,transfer the solution to another protected object glass1 and1 after evaporation1 the crystallizations characteristic of the different elements will be seen4 ,if the evaporation is too rapid and the crystallizations incomplete1 the residue should be redissolved in water1 or a very dilute solution of the solvent employed1 transferred to a fresh glass and allowed to recrystallize4 ,borichy's ,method4#adh 7,hydrofluosilicic ,acid47 ,this method has the advantage of simplicity of manipulation and relative distinctness in results2 but on the other hand these results are only obtained after several hours1 and the temperature has an influence on the crystalline forms obtained4 ,it is well to make the tests in a temperature of about #ae ,c4 ,put a spherical drop of pure hydrofluosilicic acid#adi on the fragment and leave it for some hours in damp air until the action has been sufficient1 then transfer it to a dry air bell-glass and allow evaporation and crystallization to take place4 ,for the microscopic examination the objective 7#bjj-cjj diams4 best for these observations7 can be protected with glycerine and a mica disc or thin cover-glass1 or the drop can be all evaporated and the crystals covered with liquid balsam and a cover-glass4 ,crystallizations ,obtained by ,borichy's ,method4 ,potassium4 ,from hydrofluosilicic solutions1#aej isotropic1 colorless crystals of ,k#b,si,fl#f1 in cubes1 octahedra or combinations of these forms with the rhombic dodecahedron4 ,apparently orthorhombic crystals may form from acid solutions and at a low temperature1 but if these crystals are dissolved in hot water and recrystallized they will assume the normal forms4 ,platinic chloride will produce under proper conditions sharp1 yellow octahedra of ,k#b,pt,cl#f4 ,illustration3 ,,fig4 #gh4--;,fluosilicate of potassium4#aea ,illustration3 ,,fig4 #gi4--;,fluosilicate of sodium4#aea ,sodium4 ,from hydrofluosilicic solutions1 colorless1 very weakly doubly refracting1 hexagonal crystals of ,na#b,si,fl#f1 which are generally longer the higher the percentage of sodium in the solution4 ,this test is very certain even for small amounts4 ,illustration3 ,,fig4 #hj4--;,fluosilicate of calcium4#aea ,illustration3 ,,fig4 #ha4--;,fluosilicate of magnesium4#aea ,calcium4 ,from hydrofluosilicic solutions1 monoclinic crystals of ,ca,si,fl#f #b;,h#b,o of various forms1 generally spindle-shaped1 with not very strong double refraction4 ,the crystals have seldom straight-edged boundaries and are often grouped in rosettes4 ,the addition of dilute ,h#b,,so#d decomposes the crystals1 recrystallization yielding long prismatic crystals of gypsum 7distinction from strontium74 ,treatment with ,h,fl and dilute ,h#b,,so#d 7in excess71 producing on evaporation characteristic crystals of gypsum1 furnishes a very delicate test for small percentages of calcium4 ,magnesium4 ,from hydrofluosilicic solutions1 rhombohedral crystals of ,mg,si,fl#f #f;,h#b,o with plane faces and sharp edges4 ,the crystals are colorless and strongly doubly refracting with positive optical character1 ,fig4 #ha4 ,the formation of struvite crystals 7,,nh#d,mg,p,o#d #f;,h#b,o71 of coffin-like forms1 is very characteristic and takes place from very dilute solutions 7rendered alkaline7 on the addition of a grain of salt of phosphorus or a drop of sodium phosphate4 ,iron4 ,from hydrofluosilicic solutions1 crystals of ,fe,si,fl#f #f;,h#b,o1 which are isomorphous with those of magnesium salts1 with the same optical characters4 ,they may be differentiated by moistening with potassium ferrocyanide or ammonium sulphide1 in the first case by turning blue1 in the second case black4 ,aluminium4 ,from hydrofluosilicic solutions1 not satisfactory on account of the gelatinous formation4 ,when the gelatinous formation is obtained by the action of hydrofluoric acid on an aluminous silicate1 the staining test can be used to distinguish between fine grains of feldspar and quartz or iolite and quartz4 ,behrens' ,method4#aeb 7,hydrofluoric and ,sulphuric ,acids47 ,this method depends on common reactions that can be made rapidly1 but has the disadvantage of being rather complicated and requiring delicate manipulation4 ,the tests are best made upon about mg4 of powder with ,h,fl 7pure and fuming74 ,as soon as the fluorides begin to dry treat with dilute ,h#b,,so#d and warm until white fumes of ,,so#c appear4 ,in this way the ,h,fl and ,si,fl#d are driven off and the sulphates are left4 ,this part of the test can conveniently be made on a piece of platinum foil4 ,add excess of water and concentrate4 ,transfer a drop to a clean object glass and1 while still liquid1 examine it with the microscope4 ,do not use a cover-glass over the drop4 ,crystallizations ,obtained by ,behrens' ,method4 ,potassium4 ,add a little platinic chloride when octahedral crystals of ,k#b,pt,cl#f 7size 4#ah to 4#cj mm47 will appear1 which are clear1 bright yellow in color with strong refraction4 ,illustration3 ,,fig4 #hb4--,potassium platinic chloride4#aec ,illustration3 ,,fig4 #hc4--,sulphate of calcium 7gypsum74#aec ,sodium4 ,use sulphate of cerium and allow a small amount of this reagent to act through a capillary pipette upon a drop of the solution4 ,very small aggregates of brown crystals 7size 4#jb mm47 of the double sulphate of cerium and sodium are formed1 which are clearly visible with #fjj diams4 ,if potassium is present the double sulphate of that alkali will appear in larger1 grayish grains 7size 4#je to 4#jf mm474 ,an excess of ,h#b,,so#d is to be avoided4 ,calcium4 ,after a few minutes little gypsum crystals 7,ca,s,o#d #b;,h#b,o7 will appear4 ,in strongly acid solutions the thin acicular crystals will be grouped in bushes or stars2 in neutral solutions the crystals will have the normal shape of selenite crystals or form swallow-tailed twins4 ,magnesium4 ,use salt of phosphorus dissolved in water and allow it to mix with the solution 7to which has been added ammonium chloride and ammonia7 through a capillary pipette4 ,from a solution containing more than #e per cent4 of magnesium are first deposited ,x-shaped skeletons and rudimentary crystals of ,mg4,,nh#d4,,po#d #f;,h#b,o4 ,if the solution is more dilute beautiful1 sharp1 hemimorphic crystals 74#aj to 4#bj mm4 in size7 of the orthorhombic system will appear4 ,these crystals often resemble the roof of a house4 ,the formation of the crystals is assisted by heat4 ,iron and manganese phosphates yield crystals of the same type1 but the iron is separated on the addition of ammonia4 ,aluminium4 ,use chloride of csium4 ,take a drop of the solution1 with excess of ,h#b,,so#d driven off1 and touch it with a platinum wire that has been dipped in the melted chloride of csium4 ,large crystals 74#dj-4ij mm4 in size7 of csium alum will form1 which are octahedral and cubo-octahedral in shape4 ,iron does not interfere1 as its crystallization would take place much more slowly4 ,the solution should not be too concentrated4 ,illustration3 ,,fig4 #hd4--,magnesiumammonium phosphate4#aed ,illustration3 ,,fig4 #he4--;,csium alum4#aed ,special ,tests4 ,distinction between haynite 7contains ,ca,s,o#d7 and noselite 7contains ,na#b,,so#d74 ,treat with ,h,cl and on evaporation the characteristic crystals of gypsum will be seen if the mineral is haynite4 ,dilute acid should be used and as low a temperature maintained as possible1 otherwise crystals of anhydrite would form instead of gypsum4 ,recognition of apatite by test for phosphorus4 ,treat with a drop of ammonium molybdate dissolved in ,,hno#c4 ,after complete action remove the solution to a clean object glass1 when after slight warming a large number of very small yellow crystals 7rhombic dodecahedral in shape7 will form4 ,the test may be used to distinguish this mineral from nephelite1 melilite and natrolite4 ,in the presence of soluble silica evaporate to render it insoluble and treat again with ,,hno#c and the reagent4 ,other micro-chemical tests are not mentioned for the reason that in elaborate chemical investigations of sections or isolated fragments recourse should be made to the most complete publications on the subject4 ,,appendix4 ,,brief ,,scheme ,,of ,,classification ,,into ,,systems ,,by ,,optical ,,determinations4 ,,homogeneous4 ,the whole substance shows the same optical character1 except in the case of twin crystals when the different portions of the twin are affected differently4 ,isotropic ,all sections of the substance remain dark during a complete rotation between crossed nicols1 and no interference figure is produced by convergent light4 ,amorphous ,absence of crystalline form or cleavage4 ,isometric ,presence of crystalline form or cleavage4 ,anisotropic4 ,sections generally show some interference color and extinguish four times1 at #ij apart1 during complete rotation4 ,uniaxial4 ,determined by character of interference figures obtained by convergent light from sections which remain dark or nearly so during complete rotation4 ,all sections show parallel or symmetrical extinction4 ,tetragonal ,sections giving interference figures are four- or eight-sided1 or show rectangular cleavage4 ,hexagonal ,sections giving interference figures are three-1 six- or nine-sided1 or show cleavage lines intersecting at #fj4 ,biaxial ,determined by character of interference figures obtained by convergent light4 ,orthorhombic ,extinction is parallel or symmetrical in all sections parallel to 1 b and 4 ,color distribution is symmetrical to two lines and to the central point1 see p4 #dh4 ,monoclinic ,extinction is only parallel or symmetrical in sections parallel to the ortho axis b2 all other sections show extinction angles4 ,color distribution is only symmetrical to one line or to the central point1 see p4 #dh4 ,triclinic ,extinction angles in all sections1 although in some minerals these angles may be very small4 ,no symmetry in color distribution1 see p4 #dh4 ,,aggregate4 ,not homogeneous1 but made up of an aggregation of individuals1 all extinguishing at different times4 ,double ,refraction 7maximum74#aee #j4bhg ,rutile #j4agi ,dolomite #j4agb ,calcite #j4ada ,titanite #j4jij ,titanite #j4jgb ,hornblende 7basaltic7 #j4jfb ,zircon #j4jeh ,biotite #j4jej ,talc #j4jej girite #j4jda ,muscovite #j4jcg ,epidote #j4jcf ,chrysolite 7,olivine7 #j4jcf ,scapolite 7,meionite7 #j4jcd ,tourmaline #j4jcd ,phlogopite #j4jcj ,allanite 7,orthite7 #j4jbi ,diopside #j4jbg ,actinolite #j4jbg ,tremolite #j4jbd ,hornblende 7common7 #j4jbd ,diallage #j4jbd ,anthophyllite #j4jbc ,augite #j4jba ,sillimanite 7,fibrolite7 #j4jah ,glaucophane #j4jag ,tourmaline 7precious7 #j4jaf ,cyanite 7,disthene7 #j4jac ,hypersthene #j4jac ,scapolite 7,marialite7 #j4jac ,anorthite #j4jab ,natrolite #j4jaa ,chlorite 7,clinochlore7 #j4jaa ,andalusite #j4jaa ,topaz #j4jaj ,staurolite #j4jaj ,gypsum #j4jaj ,enstatite #j4jaj ,serpentine #j4jji ,corundum #j4jji ,iolite 7,cordierite7 #j4jji ,quartz #j4jjh ,topaz #j4jjh ,kaolin #j4jjh ,labradorite1 ,ab#a;,an#a #j4jjh ,oligoclase1 ,ab#d;,an#a #j4jjh ,albite #j4jjg ,orthoclase #j4jjg ,microcline #j4jjf ,vesuvianite #j4jje ,zoisite #j4jjd ,nephelite 7,elolite7 #j4jjc ,apatite #j4jjc ,melilite #j4jjb ,tridymite #j4jjb ,leucite #j4jjb ,allanite 7,orthite7 #j4jja ,chlorite 7,penninite7 #j4jja ,vesuvianite ,indices of ,refraction 7mean74#aee #b4gab ,rutile #b4ch ,perofskite #b4jj ,spinel 7,chrome7 #a4ifc ,titanite #a4ie ,zircon #a4ibj ,titanite #a4hef ,garnet 7,melanite7 #a4gib girite #a4gh ,allanite 7,orthite7 #a4gh ,garnet 7,almandite7 #a4gff ,corundum #a4gea ,epidote #a4ge ,garnet 7,pyrope7 #a4gda ,staurolite #a4gbc ,hypersthene #a4gb ,spinel #a4gbj ,zoisite #a4gbj ,cyanite 7,disthene7 #a4gai ,hornblende 7basaltic7 #a4gae ,vesuvianite #a4gaa ,augite #a4fii ,zoisite #a4fig ,diopside #a4fhh ,diallage #a4fge ,chrysolite 7,olivine7 #a4ffe ,enstatite #a4ffd ,sillimanite 7,fibrolite7 #a4fgd ,tourmaline #a4fdd ,anthophyllite #a4fd ,hornblende 7common7 #a4fcg ,andalusite #a4fce ,apatite #a4fcb ,glaucophane #a4fcb ,topaz #a4fcj ,melilite #a4fcc ,tourmaline 7precious7 #a4fbb ,dolomite #a4fba ,actinolite #a4fba ,tremolite #a4fah ,biotite #a4fjh ,topaz #a4ehh ,chlorite 7,clinochlore7 #a4ehg ,muscovite #a4ehb ,anorthite #a4egf ,chlorite 7,penninite7 #a4egb ,talc #a4ehd ,scapolite 7,meionite7 #a4efd ,phlogopite #a4ef ,serpentine #a4eei ,labradorite1 ,ab#a;,an#a #a4ee ,kaolin #a4edg ,quartz #a4eea ,scapolite 7,marialite7 #a4fja ,calcite #a4eda ,oligoclase1 ,ab#d;,an#a #a4ed ,canada balsam #a4eci ,nephelite 7,elolite7 #a4eci ,iolite 7,cordierite7 #a4ece ,albite #a4ebe ,gypsum #a4ebe ,microcline #a4ebc ,orthoclase #a4eji ,leucite #a4ejc ,haynite #a4dhh ,analcite #a4dhc ,natrolite #a4dhc ,sodalite #a4dgg ,tridymite #a4df ,opal ,illustration3 ,diagram showing relation between strength of ,double ,refraction1 ,interference ,colors and ,thickness of ,section4#aef ,,order ,,of ,,consolidation ,,of ,,the ,,constituent ,,minerals ,,in ,,plutonic ,,rocks4 8,there is in plutonic rocks a normal order of consolidation for the several constituents1 which holds good with a high degree of generality4 ,it is in the main1 as pointed out by ,rosenbusch1 a law of #8decreasing basicity4' ,the order is briefly as follows3 8#a4 ,minor accessories 7apatite1 zircon1 sphene1 garnet1 etc47 and iron ores4 8#b4 ,ferro-magnesian minerals--olivine1 rhombic pyroxenes1 augite1 girine1 hornblende1 biotite1 muscovite4 8#c4 ,felspathic minerals--plagioclase felspars 7in order from anorthite to albite71 orthoclase 7and anorthoclase74 8#d4 ,quartz1 and finally microcline4 8,in most rocks such minerals as are present follow the above order4 ,the most important exceptions are the intergrowth of orthoclase and quartz and the crystallization of quartz in advance of orthoclase in some acid rocks1 and the rather variable relations between groups #b and #c in some more basic rocks4 ,the order laid down applies in general to parallel intergrowths of allied minerals2 thus when augite is intergrown with girine or hornblende the former mineral forms the kernel of the complex crystal and the latter the outer shell2 when a plagioclase crystal consists of successive layers of different compositions the layers become progressively more acid from the center to the margin4 8,certain constituents having variable relations are omitted from the foregoing list4 ,thus nepheline 7elolite7 and sodalite belong to group #c1 but may crystallize out either before or after the feldspar40#aeg ,,optical ,,scheme4 ,,introduction4 ,the scheme is designed to furnish the student with a practical method of recognizing the common minerals in rock sections4 ,the arrangement followed has been to group minerals having general optical characters in common1 at the same time giving their specific characters so as to make it possible to distinguish one from another4 ,in each rectangle the minerals are arranged in order of their indices of refraction4 ,a tabulation of the minerals1 with a list of optical characters appended1 is of aid to the skilled investigator1 but of very little assistance to the beginner4 ,the more common minerals1 or those which are important petrographically1 are printed in heavy-faced type2 the minerals of less importance in small capitals4 ,,abbreviations ,,and ,,conventions ,,used4 ,a4 ,amorphous4 ,i4 ,isometric system4 ,t4 ,tetragonal system4 ,o4 ,orthorhombic system4 ,m4 ,monoclinic system4 ,tri4 ,triclinic system4 ,h4 ,hexagonal system4 ,m7,h74 ,monoclinic1 with hexagonal form or characters1 as in the case of biotite4 ,at right angles to4 ,el4 ,elongation4 ,ex4 ,extinction4 ,ex4 ,parallel extinction1 as when the crystals extinguish parallel to cleavage lines or crystal edges4 ,extinction which is symmetrical to intersecting cleavage lines is also included under this term4 ,the mean refractive indices are printed in heavy-faced type4 ,the term 8grains0 is used to describe not only minerals which occur in typically granular form1 but also those which have coarser allotriomorphic form1 as elolite and sodalite in plutonic rocks1 such as syenite1 etc4 ,,general ,,rules ,,for ,,use ,,of ,,scheme4 ,the division of the scheme into two vertical columns is based on the values of the mean refractive indices1 as determined by the 8relief0 and appearance of the surface4 ,when the refractive index is above #a4fj1 the relief is fairly well to distinctly marked and the surface rough to very rough1 depending on the value of the index4 ,most of the rock-forming minerals with indices below #a4fj show no relief and a smooth surface1 except in the case of a few of the rarer minerals 7mostly isometric71 which have very low indices and hence rough surface4 ,mistakes may easily be made in the case of minerals near the limit2 but practice and the use of the ,becke test should soon make possible the classification into the two groups suggested by the scheme1 and the appended descriptions will help to check errors4 ,when an unknown mineral lies adjacent to one that is known1 use the ,becke test for obtaining the relative refractive index of the unknown mineral2 focus sharply on the line of contact between the known and unknown minerals1#aeh then raise the objective slightly and the 8bright line0 will appear on the side of the mineral having the higher index4 ,the method of ,schrder van der ,kolk may also be employed1 see p4 #bb4 ,the horizontal divisions of the scheme depend on the relative strength of the double refraction based on the observed interference colors4 ,these colors to be of use in classification must be correctly recognized4 ,the lower and middle colors of the #a order1 from bluish-gray through white to yellow1 are easily known4 ,the bright red1 blue1 green1 etc41 colors of the #a1 #b and #c orders can also be differentiated without trouble from the very high order colors 7#d and above71 which are essentially white in tone with no decided color tint4 ,when confusion arises1 the exact order of the color can be determined by a quartz wedge1 as given on p4 #ce4 ,furthermore1 a undulation mica plate serves to quickly distinguish between the #a order white and the practically colorless1 high order tint of calcite1 titanite1 etc42 as after the insertion of the test-plate the #a order white suffers a marked change in color1 while the very high order 7practically white7 tint shows no appreciable change4 ,in the determination of these interference colors care must be given to considerations of orientation and thickness4 ,the section must give the maximum interference color of all the obtainable sections of the mineral in the rock section4 ,such sections will be parallel to the axis in the uniaxial minerals and to the axial plane in the biaxial minerals2 and will1 therefore1 in convergent light never show the emergence of an optic axis or bisectrix4 ,crystal 8form10 cleavage1 pleochroism1 etc41 may at times aid in the selection of these sections4 ,the thickness of the section must also be considered1 and it is well in all cases to pick out some known mineral in the section1 as quartz1 and note its maximum interference color4 ,knowing how this varies from the color given by the scheme for a section #j4jc mm4#aei in thickness1 due allowance can be made for a like variation in the colors given by the other minerals4 ,in the case of minerals with strong absorption the interference colors may not be noticeable on account of the absorption of parts of the light4 ,under the subhead of pleochroism1 the vibration direction of the ray of a definite color is given and not the direction of transmission of that ray4 ,the interference figures in convergent light increase in clearness and distinctness with the strength of the double refraction4 ,in uniaxial crystals sections at right angles to the optic axis1 i4 e41 sections which remain dark during a revolution between crossed nicols1 show the best interference figures4 ,in biaxial crystals the most characteristic interference figures are shown by sections at right angles to the acute bisectrix4 ,crystal sections which are too small do not give very satisfactory interference figures with convergent light4#afj ,any scheme1 however designed1 makes a more or less arbitrary classification of the minerals1 and when in doubt it is always safer to look for the mineral on both sides of the scheme line4 ,the rarer minerals are not included in this scheme1 so when the determination of a mineral is uncertain or not positive recourse should be had to more elaborate tables4 ,,index4 ,,mineral ,,names ,,in ,,heavy-,,faced ,,type4 ,abbreviations1 ix ,abnormal interference colors1 #cj ,absorption1 #bf directions1 #bf tints1 #ag ,acmite1 #hc ,actinolite1 #hd ,acute bisectrix1 #e girine1 #hc girine-augite1 #hd ,aggregate structure1 #ch ,albite1 #ajc ,allanite1 #if ,allotriomorphic1 #ae ,aluminium1 test for1 by ,borichy's method1 #acg test for1 by ,behren's method1 #aci ,amorphous bodies1 #b ,amorphous substances1 test for1 #bf ,amphibole1 #hd ,analcite1 #ef ,analyzer1 #aj ,andalusite1 #gb ,anhedron1 #ae ,anisotropic character1 test for1 #bg ,anisotropic crystals1 #b ,anomalies1 optical1 #bg ,anorthite1 #ajc ,anorthoclase1 #aac ,anthophyllite1 #hh ,antigorite1 #aad ,apatite1 #fh ,apatite1 recognition of1 by test for phosphorus1 #aci ,apparatus for petrographical laboratory1 #abg ,appendix1 #ada ,arfvedsonite1 #hh ,augite1 #hj ,automorphic1 #ac ,axes of elasticity1 #d ,axial angle1 #e determination of1 #dg ,axial plane1 #e ,basaltic ,hornblende1 #he ,bastite1 #gg ,becke's method for determining relative values of refractive indices1 #ai ,behren's method4 7,hydrofluoric and sulphuric acids71 #acg ,bertrand lens1 #aa ,biaxial crystals1 #d1 #e vibration directions in1 #d ,biaxial interference figures1 #dc ,biotite1 #hi ,bisectrix1 acute1 #e obtuse1 #e ,borichy's method4 7,hydrofluosilicic acid71 #ace ,broken or strained crystals1 #ae ,bronzite1 #gf ,calcite1 #fe ,calcium1 test for1 by ,borichy's method1 #acf test for1 by ,behren's method1 #ach 8,cap0 nicol1 #aj ,carbonaceous ,matter1 #fa ,carbonaceous particles1 test for1 #acb ,carbonates1 test for1 #acj ,cement1 #abb for mounting1 #abf ,cementing1 #aba ,centering stage1 #aj ,chalcedony1 #fd ,characters observed by1 convergent light1 #dj crossed nicols1 #bf reflected light1 #ac polarized light1 #be ,characters of opaque minerals1 #ac transparent minerals1 #ac ,chemical and mechanical tests1 #abi ,chemical 7micro7 reactions1 #acd separation1 #acd tests on crystal in section1 #abi ,chiastolite1 #gb ,chlorite group1 #ib ,chromite1 #eb ,chrysolite1 #gg ,chrysotile1 #aad ,circular polarization1 #c ,classification into systems by optical determinations1 #ada ,clay1 #aae ,cleaning and finishing sections1 #abf ,cleaning microscope1 #aa ,cleavage1 #bc ,clinozoisite1 #if ,color1 #ag diagram 7interference71 #adc fringes1 #dh ,condensing lens1 #i ,consolidation of minerals in plutonic rocks1 order of1 #add ,conventions1 ix ,convergent light1 #dj characters observed by1 #dj ,convergent and parallel light1 rsum of uses of1 #di ,cordierite1 #gh ,corroded crystals1 #ae ,corundum1 #fb ,crossed nicols1 characters observed by1 #bf ,crossed twinning1 #cg ,crystallites1 #af ,crystallizations obtained by ,behren's method1 #acg ,borichy's method1 #ace ,crystals bounded by planes1 #ac ,crystals without bounding planes1 #ae ,cutting1 #abj ,cutting and grinding machines1 #aag ,cyanite1 #aac ,damourite1 #ib ,delessite1 #id ,diagram1 showing relation between strength of double refraction1 interference colors and thickness of section1 #adc ,diallage1 #hj ,diamond saws1 #aai ,dichroite1 #gh ,diopside1 #hj ,dipyre1 #ei ,directions of elasticity1 ix ,dispersion1 #e1 #dh ,disthene1 #aac ,dolomite1 #fg ,double image1 #b ,double refraction1 #b1 #bg estimation of strength of1 #cj strength of1 measured by von ,federow mica wedge1 #ce table 7maximum71 #adb ,effects produced by crystals on transmitted light1 #b ,elolite1 #fi ,elasticity1 axes of1 #d directions of1 ix ,electric motor for grinding machines1 #aag ,electro-magnetic separation1 #acd ,emery1 grades of1 for grinding1 #abd ,enstatite1 #ge ,epidote1 #id ,etched figures1 #acj ,extinction1 #ca angles1 #e1 #ca tests for1 #cb wavy1 #ca ,extraordinary ray1 #c ,eye-pieces1 #aa ,faster and slower rays1 test for vibration directions of1 #cc ,fayalite1 #gh ,feldspar ,group1 #ih ,feldspathoids1 #fi ,fibrolite1 #gc ,finishing and cleaning sections1 #abf ,focusing1 #aa ,form of crystals1 #ac ,fracture1 #bd ,garnet1 #ec ,gelatinizing silica1 test for1 #acj ,glass slides1 #abe ,glaucophane1 #hh ,graphite1 #fa ,gravity 7specific7 separation1 #acc ,gridiron twinning1 #cg ,grinding1 #abc ,grinding and cutting machines1 #aag apparatus for sections with parallel faces1 #aah plates or laps1 #aba ,gypsum1 #hj ,gypsum test-plate1 #cc ,hardness of grains1 #acb ,hayne1 #ef ,haynite1 #ef ,haynite and noselite1 micro-chemical distinction between1 #aci ,heating sections to redness1 #aca ,heavy solutions1 #acc ,hematite1 #fa ,hexagonal crystals1 #d ,high relief1 #ag ,hornblende1 #hd ,hyalosiderite1 #gh ,hydrargillite1 #aae ,hydrofluosilicic acid test1 #ace ,hypersthene1 #ge ,idiomorphic1 #ac ,idocrase1 #fj ,ilmenite1 #fb ,inclusions1 #bd ,index of refraction1 ix1 #ag by ,becke's method1 #ai by ,schrder van der ,kolk's method1 #bb by ,sorby's method1 #ai ,indicators for specific gravity separation1 #acc ,indices of refraction 7mean71 table of1 #adb ,interference color1 determination of order of1 #ce ,interference colors1 #bh ,interference colors 7abnormal71 #cj ,interference figures1 #dj biaxial1 #dc uniaxial1 #dj ,investigation of microscopic and optical characters of minerals1 #ac ,iolite1 #gh ,iron1 test for1 by ,borichy's method1 #acg test for1 by ,behren's method1 #aci ,isolating crystals or mineral fragments for testing1 #acb ,isometric crystals1 #b ,isotropic character1 #bf1 #dj crystals1 #b ,kaolin1 #aae ,labradorite1 #ajc ,laps for grinding1 #aba ,leucite1 #ed ,leucoxene1 #fb ,light1 ordinary1 #a plane polarized1 #a ,limonite1 #ea ,lithia ,mica1 #ib ,machines for cutting and grinding1 #aag ,magnesium1 test for1 by ,borichy's method1 #acg test for1 by ,behren's method1 #ach ,magnetic iron ore1 #eb ,magnetic pyrites1 #ea ,magnetite1 #eb ,measuring strength of double refraction by von ,federow mica wedge1 #ce ,mechanical and chemical tests1 #abi ,melilite1 #fa ,menaccanite1 #fb ,methods of preparing sections1 #aag ,mica ,group1 #hi ,mica plate 7quarter undulation71 #cc ,mica wedge1 von ,federow1 #ce ,micro-chemical reactions1 #acd ,microcline1 #ajb ,microlites1 #ag ,microscope 7petrographical71 #g ,microscopic and optical characters of minerals1 #ea ,minerals and thickness of section1 determined by table and diagram1 #cf ,monoclinic crystals1 #d principal vibration directions in1 #e ,monoclinic pyroxenes1 #hj ,mounting sections1 #abd solutions1 #abe ,muscovite1 #hi ,natrolite1 #gi ,negative optical character1 biaxial1 #df uniaxial1 #db ,nepheline1 #fi ,nephelite1 #fi ,nicol prism1 #g ,nosean1 #ef ,noselite1 #ef ,noselite and haynite1 micro-chemical distinction between1 #aci ,nose-piece for microscope1 #aj ,objectives1 #aj ,oblique extinction1 #ca ,obtuse bisectrix1 #e ,oligoclase1 #ajc ,olivine1 #gg ,opal1 #ea ,opaque minerals1 characters of1 #ac ,optical1 anomalies1 #bg character1 biaxial1 #df character1 uniaxial1 #db classification into systems1 #ada distinctions between orthorhombic1 monoclinic and triclinic sections 7perpendicular to bisectrices71 #dh1 #ada distinctions between tetragonal and hexagonal sections 7perpendicular to optic axis71 #ada and microscopic characters of minerals1 #ea principal section1 #c1 #d scheme1 #ade ,optic axes1 #d ,optic axis1 #c ,optics for optical mineralogy1 #a ,order of consolidation of minerals in plutonic rocks1 #add ,order of interference color1 determination of1 #ce ,ordinary light1 #a ray1 #c ,orthite1 #if ,orthoclase1 #ih ,orthorhombic crystals1 #d principal vibration directions in1 #d ,orthorhombic pyroxenes1 #ge ,parallel and convergent light1 rsum of uses of1 #di ,parallel extinction1 #ca ,parallel face sections1 grinding apparatus for1 #aai ,pargasite1 #he ,perofskite1 #eg ,perovskite1 #eg ,petrographical apparatus1 #abg ,petrographical microscope1 #g ,phenocryst1 #ad ,phlogopite1 #hi ,picotite1 #ec ,piedmontite1 #ie ,plagioclases1 #ajc ,plane polarized light1 #a ,plates1 grinding1 #aba ,pleochroic halos1 #ei ,pleochroism1 #be test for1 #be ,pleonaste1 #ec ,polarization colors1 #bh ,polarized light1 characters observed by1 #be plane1 #a ,polarizer1 #g test for vibration plane of1 #i ,polishing sections1 #abd ,polysynthetic twinning1 #cg ,positive optical character1 biaxial1 #df uniaxial1 #db ,potassium1 test for1 by ,borichy's method1 #acf test for1 by ,behren's method1 #ach ,preparation of sections1 #aag ,principal section1 optical1 #c1 #d ,principal vibration directions1 #c1 #d ,prism1 nicol1 #g ,pseudomorphic structure1 #ci ,pyrite1 #ea ,pyrites1 #ea ,pyroxenes1 monoclinic1 #hj ,pyroxenes1 orthorhombic1 #ge ,pyrrhotite1 #ea ,quarter-undulation mica plate1 #cc ,quartz1 #fc wedge1 #cd ,reflected light1 characters observed by1 #ac ,reflector1 #g ,refraction1 double1 #b1 #bg ,refraction1 index of1 ix1 #ag ,refractive indices 7mean71 table of1 #adb ,relief1 #ag test for1 #ag1 #ah ,resorption border1 #ae ,rsum of uses of parallel and convergent light1 #di ,rotating stage of microscope1 #aj ,rutile1 #eg ,sagenite1 #eh ,sanidine1 #ajb ,saussurite1 #if1 #aac ,saws1 #aai ,scapolite ,group1 #ei ,scheme of classification into systems by optical determinations1 #ada ,scheme1 optical1 #ade ,schiller structure1 #be ,schrder van der ,kolk's method for determining refractive indices1 #bb ,sections1 methods of preparing1 #aag thickness of rock1 #abd ,selenite plate1 #cc ,separation1 by chemical means1 #acd by electro-magnet1 #acd by specific gravity1 #acc ,sericite1 #ib ,serpentine1 #aad ,shagreened surface1 #ag ,shorl1 #gj ,sillimanite1 #gc ,simple twinning1 #cg ,single image1 #b ,skeleton forms1 #ag ,slides1 glass1 #abe ,slower and faster rays1 test for vibration directions of1 #cc ,sodalite1 #ef ,sodalite ,group1 #ef ,sodium1 test for1 by ,borichy's method1 #acf test for1 by ,behren's method1 #ach ,sorby's method for determining index of refraction1 #ai ,special micro-chemical tests1 #aci ,specific gravity separation1 indicators for1 #acc ,sphrulitic structure1 #ci ,sphene1 #if ,spinel1 #ec ,stage of microscope1 rotating1 #aj ,staurolite1 #gd ,strained or broken crystals1 #ae ,strength of double refraction1 measure of1 #cj1 #ce ,structure1 #cg ,symmetrical extinction1 #ca ,systems1 classification into1 by optical determinations1 #ada ,table1 double refraction1 #adb ,table1 indices of refraction1 #adb ,talc1 #id ,test for vibration plane of polarizer1 #i ,tetragonal crystals1 #d ,thickness of rock sections1 #abd ,thickness of section and minerals1 determined by table and diagram1 #cf ,titanite1 #if ,transmitted light1 characters observed by1 #ac effect on crystals1 #b ,transparent minerals1 investigation of characters of1 #ac ,tremolite1 #hd ,triclinic crystals1 #d principal vibration directions in1 #e ,tridymite1 #fe ,topaz1 #gd ,tourmaline1 #gj ,twinning1 #cg ,uniaxial crystals1 vibration directions in1 #c ,uniaxial interference figures1 #dj ,uralite1 #hh ,uralitization1 #hh ,uses of parallel and convergent light1 #di van der ,kolk's1 ,schrder1 method for determining refractive indices1 #bb ,vesuvianite1 #fj ,vibration direction1 #b directions of faster and slower rays1 test for1 #cc directions1 principal1 #c1 #d plane of nicol1 #i plane of polarizer1 test for1 #i von ,federow mica wedge1 #ce ,wavy extinction1 #af1 #ca ,wedge1 mica1 #ce quartz1 #cd ,wernerite1 #ei ,xenomorphic1 #ae ,zeolites1 #gi ,zircon1 #eh ,zoisite1 #if and 1 #if ,zonal structure1 #ch ,,errata ,,and ,,addenda4 ,p4 #cc1 line #bc1 for beat read be at4 ,p4 #ff1 line #aj1 after appearance add 7sometimes called 8twinkling074 ,corrections for mean index of refraction n3 p4 #fj1 line #a1 #a4eea to #a4ehd2 p4 #fb1 line #bi1 #a4gff2 p4 #ff1 line #c1 #a4fja2 p4 #fg1 line #bd1 #a4fbb2 p4 #ga1 line #aj1 #a4fcc to #a4fgd4 ,corrections for ,indices of ,refraction 7mean7 in ,table on p4 #adb3 ,corundum #a4gff2 ,tourmaline #a4fgd2 ,tourmaline 7precious7 #a4fcc2 ,dolomite #a4fbb2 ,scapolite 7,meionite7 #a4ehd2 ,scapolite 7,marialite7 #a4eea2 ,calcite #a4fja4 ,corrections for mean indices of refraction in ,scheme 7insert folder73 ,scapolite #a4eea to #a4ehd2 ,calcite #a4fja and transfer to ,dolomite rectangle2 ,dolomite #a4fbb2 ,corundum #a4gff2 ,tourmaline #a4fcc to #a4fgd4 ----- ,footnote #a3 ,often called a direction of maximum elasticity1 c being a direction of minimum elasticity4 ,footnote #b3 ,rosenbusch's ,microskopische ,physiographie1 p4 #aef4 ,footnote #c3 ,for a more complete discussion of optics1 in connection with ,optical ,mineralogy1 the student is referred to ,a4 ,j4 ,moses' ,characters of ,crystals1 p4 #he1 et seq42 ,moses' ,parsons' ,mineralogy1 ,crystallography and ,blowpipe ,analysis1 ,chap4 ,,xvi41 #dth ,ed41 #aiji2 ,miers' ,mineralogy1 #aijb2 ,l4 ,fletcher's ,optical ,indicatrix1 etc41 #ahib2 ,groth's ,physikalische ,krystallographie1 #c;d ,ed42 ,rosenbusch's ,mikroskopische ,physiographie1 #dth ,ed4 and ,iddings' ,rock ,minerals1 #aiaa4 ,footnote #d3 ,the electromagnetic theory of light is now very generally held1 but1 whatever may be the recurrent change of state to which light is really due1 the principles of wave motion furnish a satisfactory geometric description of optical phenomena4 ,footnote #e3 ,these sections are supposed to have plane parallel faces1 such being the case in ordinary practice1 and to be examined with parallel perpendicularly incident light4 ,footnote #f3 ,it is interesting to remember in this connection that in the isometric system there is also the greatest possible symmetry of 8form40 ,footnote #g3 ,a4 ,j4 ,moses1 ,characters of ,crystals1 pp4 #he-ig4 ,footnote #h3 ,for this branch of optical physics1 see ,a4 ,j4 ,moses1 ,characters of ,crystals1 pp4 #ig-ajj4 ,footnote #i3 ,this can be demonstrated by using a nicol and a plate of calcite which shows a double image4 ,if the nicol is held between the calcite plate and the observer's eye it can be so adjusted that only one image is seen4 ,if now the nicol is revolved #ij the first image will disappear and the other image alone will be seen4 ,footnote #aj3 ,in some cases a peculiar form of double refraction does take place parallel to this direction1 as in the circular polarization of quartz and cinnabar2 but in very thin sections these results are not noticed and can be disregarded4 ,footnote #aa3 ,a4 ,j4 ,moses1 ,characters of ,crystals1 pp4 #ih1 #ii4 ,footnote #ab3 ,the terms axes of elasticity are commonly used for these principal vibration directions in text-books on petrography4 ,footnote #ac3 ,instead of and 1 for convenience in tables1 etc41 and are used1 denoting the indices of refraction of the rays traversing the crystal with greatest and least velocity respectively1 without regard as to which is the ,o or ,e ray4 ,a good reason for this convention is that the symbol 7 7 is used to express in decimals the relative strength of the double refraction of a crystal1 whether uniaxial or biaxial4 is always greater than 4 ,footnote #ad3 ,for most cases in observations with white light the 8optics axes0 may be regarded as approximately fixed in position4 ,footnote #ae3 ,often spoken of as3 a1 the axis of maximum2 b1 the axis of intermediate1 and c1 the axis of minimum elasticity4 ,in the more recent ,american text-books on ,optical ,mineralogy by ,iddings1 ,winchell and ,phillips a is denoted by ,x1 b by ,y and c by ,z4 ,footnote #af3 ,for a short historical sketch of the use of the microscope in connection with ,petrology1 see ,g4 ,h4 ,william's pamphlet1 ,modern ,petrography 7,monographs of ,education71 ,boston1 #ahhf4 ,for detail description and adjustments1 see ,methods of ,petrographic-,microscopic ,research1 ,f4 ,e4 ,wright1 #aiaa1 pp4 #aa1 #fa4 ,footnote #ag3 ,for description of the ordinary microscope1 eye-pieces1 objectives1 magnification1 etc41 see ,manipulation of the ,microscope1 by ,ed4 ,bausch4 ,footnote #ah3 ,text ,book of ,mineralogy1 by ,e4 ,s4 ,dana1 #ahih ,ed41 p4 #agf4 ,footnote #ai3 ,it is convenient to assume that the vibrations of the polarized light are taking place in this plane1 called the 8plane of vibration10 but all the phenomena caused by polarized light could be also explained on the assumption that the vibrations were taking place at right angles to this plane4 ,footnote #bj3 ,this condensing lens must be removed when very low power objectives are used4 ,footnote #ba3 ,some microscopes are provided with adjusting screws bearing on the frame holding the objective1 which can then be accurately centered to the axis of rotation of the stage4 ,footnote #bb3 ,in the ,seibert microscope use objective ,no4 #jj for the first general study of a rock section1 ,no4 ,,ii for general use and ,no4 ,v for observations with convergent light4 ,in the ,fuess microscope use objective ,no4 #d for general use and ,no4 #g for convergent light tests4 ,in the case of an ,english microscope a #a to objective is used for general purposes and a to for observations with convergent light4 ,footnote #bc3 ,see pp4 #cc and #cd4 ,footnote #bd3 ,in some microscopes the analyzer is in the form of a 8cap0 nicol1 arranged to be fitted over the top of the eye-piece1 and not introduced in the microscope tube as shown here4 ,this form is not so convenient1 as the 8cap0 nicol must be set by hand every time it is desired to make observations with crossed nicols4 ,but at the same time it avoids any possible refocusing which may be necessary when the other type of analyzer is introduced in the tube4 ,footnote #be3 ,when accurate adjustments are possible of the vibration planes of the nicols and the cross-wires parallel to these vibration planes1 reference should be made to ,methods of ,petrographic-,microscopic ,research1 p4 #fa1 #aiaa1 ,fred4 ,e4 ,wright4 ,footnote #bf3 ,in the ,seibert microscope eye-piece ,no4 #j is used for most purposes4 ,other eye-pieces1 ,nos4 #a and #b1 with cross-wires1 are used for different degrees of magnification1 and one eye-piece1 ,no4 #c1 without cross-wires1 is provided to be used in connection with an eye-piece micrometer4 ,footnote #bg3 ,with the ,seibert microscope1 ,fig4 #b1 the ,no4 #j eye-piece and the ,no4 ,,ii objective will prove most satisfactory for the following tests4 ,with the ,fuess microscope use ,no4 #d objective4 ,with an ,english microscope use an ordinary eye-piece and a #a or objective4 ,footnote #bh3 ,twins may be recognized just as in macroscopic specimens1 and zonal structure noticed if the zones differ in color4 ,when a colorless mineral is surrounded by other colorless minerals1 of about the same index of refraction1 its outline is often best brought out by observation between crossed nicols4 ,footnote #bi3 ,the term 8,anhedron10 meaning without planes1 has been suggested by ,l4 ,v4 ,pirsson to describe in rocks the crystal fragments which have no plane faces1 as1 for example1 the augites of augitic rocks4 ,science1 ,jan4 #aj1 #ahif1 p4 #di4 ,footnote #cj3 ,partial resorption and recrystallization may produce a border of secondary minerals1 surrounding the original crystal4 ,footnote #ca3 ,the surfaces of all minerals in sections are more or less rough1 but this roughness is only made visible when there is a marked difference between the indices of refraction of the minerals1 and the index of refraction of the balsam in which the minerals are embedded4 ,the index of refraction of balsam is about #a4ed1 so it is only when the mineral has a higher or lower index of refraction that its surface appears rough4 ,when internal structure is to be studied1 the crystal should be surrounded by a fluid of nearly the same index of refraction as that of the crystal1 and when the exterior of the crystal is to be studied1 then a fluid should be used with a very different index of refraction4 ,footnote #cb3 ,the ,seibert microscope has a very convenient and quick lowering adjustment1 by means of the lever d1 for making this test4 ,for convenience the lower nicol and condensing lens are generally left in place below the stage of the microscope1 as polarized light serves as well for these investigations as ordinary light4 ,an additional advantage in this arrangement is that the condensing lens is always ready for the 8relief0 test and the lower nicol for the pleochroism test2 but it must be remembered that the polarizer or lower nicol cuts out one half of the light1 which comes to it from the reflector1 and this loss is important when high power objectives are to be used4 ,when very low power objectives are used1 the condensing lens must be removed4 ,footnote #cc3 ,mem4 de l',acad41 ,paris1 #agfg-fh4 ,footnote #cd3 ,min4 ,mag41 ,vol4 ,i41 p4 #aic2 ,vol4 ,,ii41 p4 #a4 ,footnote #ce3 ,sitzungsberichte der k4 k4 ,akad4 der ,wiss41 ,wien1 #ahic1 ,i ,abt41 p4 #ceh4 ,translation by ,l4 ,mc,i4 ,luquer in ,school of ,mines ,quarterly1 ,vol4 ,,xxiii41 ,jan41 #aijb1 ,no4 #b1 p4 #abg4 ,review by ,viola in ,min4 ,pet4 ,mitt41 ,vol4 #ad1 p4 #eed4 ,methods of ,petrographic-,microscopic ,research1 ,f4 ,e4 ,wright1 #aiaa1 p4 #ie4 ,footnote #cf3 ,most of the petrographical microscopes carry over the polarizer a convex lens the effect of which is to widen the illuminating cone and hence make less visible this phenomenon4 ,footnote #cg3 ,in ,seibert student microscope1 ,no4 ,,ii a1 use next to smallest light-stop4 ,some of the ,fuess microscopes are supplied with an iris-blende for limiting the cone of light4 ,footnote #ch3 ,in ,seibert microscope use ,no4 ,v1 not sufficiently marked results being obtained with ,no4 ,,ii4 ,footnote #ci3 ,see ,viola's diagram1 ,minn4 ,pet4 ,mitt41 ,vol4 ,,xiv41 p4 #eef4 ,footnote #dj3 ,seibert1 ,no4 ,,ii4 ,fuess1 ,no4 #d4 ,footnote #da3 ,kurze ,anleitung zur mikrosk4 ,krystallbestimmung1 ,wiesbaden1 #ahih1 and ,tabellen zur mikrosk4 ,bestimmung1 etc41 ,wiesbaden1 #aijj4 ,methods of ,petrographic-,microscopic ,research1 ,f4 ,e4 ,wright1 #aiaa1 p4 #ic4 ,this method is particularly favorable for the accurate determination of the refractive indices of small isolated fragments or grains1 by using liquids of known indices4 ,in this case a bright line appears on the 8near0 edge and a dark line on the 8far0 edge if the grain has a higher index than the liquid4 ,the reverse occurs when the index is lower than the liquid4 ,when the index is the same 7using white light7 the 8far0 edge is blue and the 8near0 edge red and also the contours about disappear4 ,f4 ,krantz1 of ,bonn1 furnishes a series of #ba liquids in small bottles1 with indices from #a4ddg to #a4hc4 ,the indices of a few convenient liquids are3 water1 #a4cd2 alcohol1 #a4cf2 glycerine1 #a4da2 olive oil1 #a4dg2 nut oil1 #a4ej2 clove oil1 #a4ed2 aniseed oil1 #a4eh2 almond oil1 #a4fj2 cassia oil1 #a4fc2 monobromnapthalene1 #a4fe2 methylene iodide1 #a4ge4 ,footnote #db3 ,crystals that have two good cleavages often develop so that the direction of elongation is parallel to the intersection of the two cleavages1 while in the case of crystals with one good cleavage the tendency seems to be towards a tabular habit parallel to the cleavage4 ,footnote #dc3 ,harker's ,petrology for ,students1 p4 #cjf4 ,footnote #dd3 ,the lower nicol is generally so adjusted that its plane of vibration is parallel to the north and south cross-wire in the eye-piece4 ,this adjustment can be tested by means of a section of biotite1 showing cleavage cracks4 ,when the plane of vibration of the polarizer is parallel to the ,n4 and ,s4 cross-wire in the eye-piece1 the biotite section becomes almost dark when its cleavage cracks are parallel to the same cross-wire4 ,the upper nicol1 or analyzer1 must1 of course1 be removed during this test4 ,this method is more convenient than taking the nicol out of its frame1 in order to ascertain its plane of vibration 7the direction of its shorter diagonal74 ,footnote #de3 ,although the 8absorption directions0 may not necessarily coincide with the principal vibration directions in ,monoclinic and ,triclinic crystals2 still for convenience the absorption colors are usually given for the light rays vibrating parallel to these principal vibration directions4 ,footnote #df3 ,in the ,fuess and ,seibert microscopes the analyzer or upper nicol is so fitted that it slides in and out of the tube of the microscope with its plane of vibration always at right angles to the plane of vibration of the polarizer or lower nicol4 ,footnote #dg3 ,moses' ,characters of ,crystals1 p4 #ajf4 ,moses and ,parsons' ,min4 ,cryst4 and ,b4 ,p4 ,analysis1 p4 #afc4 ,footnote #dh3 ,iddings' ,rock ,minerals1 #aiaa1 p4 #agb4 ,footnote #di3 ,these sections always contain the principal vibration directions a and c4 ,footnote #ej3 ,methods of ,petrographic-,microscopic ,research1 ,f4 ,e4 ,wright1 #aiaa1 p4 #aja4 ,footnote #ea3 ,a chart of interference colors can be obtained from ,baudry et ,cie1 ,paris1 and is also published in ,les ,minraux des ,roches1 by ,lvy and ,lacroix1 ,rock ,minerals 7#aiaa71 by ,iddings and in ,rosenbusch's ,mikroskopische ,physiographie4 ,footnote #eb3 ,iddings' ,rock ,minerals1 #aiaa1 pp4 #ada1 #ahc4 ,footnote #ec3 ,methods of ,petrographic-,microscopic ,research1 ,f4 ,e4 ,wright1 #aiaa1 p4 #acb4 ,footnote #ed3 ,the undulation mica plate consists of a thin cleavage of mica on which is marked c1 the vibration direction of the slower ray1 which in mica is the line joining the 8optic axes40 ,the thickness is such that the slower ray is wave-length behind the faster and the interference color is a bluish-gray4 ,the gypsum plate is a thin cleavage of gypsum1 on which is usually marked a1 the vibration direction of the faster ray4 ,the chosen thickness is such as to produce the red interference color of the #a order4 ,footnote #ee3 ,the test-plates are generally introduced in the slot k1 in a microscope of the ,seibert type1 or if a cap-nicol is used in a slot below this4 ,in case no provision is made by the instrument maker for these test-plates1 the regular analyzer is left out of the tube1 and a simple nicol prism is used as an analyzer and is held by the observer over the eye-piece4 ,care must be taken to have the plane of vibration of this nicol at right angles to that of the polarizer1 and to leave sufficient room for the introduction1 by hand1 of the test-plate between the eye-piece and the nicol4 ,with care the plates can be introduced with sufficient accuracy to make the test practical4 ,footnote #ef3 ,a scale or chart of interference colors1 or the interference color diagram1 should be before the observer in order to avoid any mistakes as to whether the new color is higher or lower in the scale4 ,footnote #eg3 ,the quartz wedge is cut so that one of its faces is exactly parallel to the axis 7hence also parallel to the c vibration direction7 while the other face makes a very small angle with it4 ,the direction c is marked on the wedge4 ,footnote #eh3 ,described under next test4 ,footnote #ei3 ,in applying this rule count the #a order white as green and the #a order gray as blue4 ,footnote #fj3 ,a4 ,j4 ,moses1 ,trans4 ,n4 ,y4 ,acad4 ,sci41 ,vol4 ,,xvi41 p4 #ee1 ,jan41 #ahig4 ,footnote #fa3 ,e4 von ,federow1 ,zeit4 f4 ,kryst41 etc41 ,vol4 ,,xxxv41 p4 #cdj1 #ahie4 ,footnote #fb3 ,after the first rough determination of the phase difference by the mica wedge1 the more exact phase difference can be obtained by the aid of a good color chart or diagram1 see end of book4 ,footnote #fc3 ,it is not safe to use minerals near the edge of the section1 as the thicknesses are apt to be unequal4 ,footnote #fd3 ,see at end of appendix4 ,footnote #fe3 ,in this way eliminate1 so far as possible1 the effect of the orientation of the mineral section4 ,footnote #ff3 ,the different mineral sections are all supposed to have the same thickness throughout the rock section4 ,footnote #fg3 ,harker's ,petrology for ,students1 #ahie1 p4 #ad4 ,footnote #fh3 ,iddings' ,rock ,minerals1 pp4 #aec1 #agc4 ,moses' ,characters of ,crystals1 p4 #aae4 ,footnote #fi3 ,in the ,seibert microscope use ,no4 ,v objective1 in ,fuess microscope ,no4 #g objective1 and in ,english microscopes a or objective4 ,footnote #gj3 ,each convergent ray will have its vibration direction either in or at #ij to the plane through the ray and the optic axis4 ,hence all rays vibrating parallel to the vibration planes of both nicols will be completely cut out4 ,as the section is rotated new rays successively come into these positions1 so the same effect is maintained4 ,footnote #ga3 ,in the ,seibert microscope there is a little slot k for this purpose just above the objective4 ,footnote #gb3 ,the optical character may also be determined in parallel light by proving c771 a774 ,the optical character of the principal zone or the sign of the elongation is often given in tables4 ,this optical character or sign is 77 when the principal zone axis or the direction of elongation is parallel to c and 77 when parallel to a4 ,footnote #gc3 ,iddings' ,rock ,minerals1 #aiaa1 p4 #agb4 ,footnote #gd3 ,this assumes the optic axes for different colors to emerge about at the same points4 ,if there is marked 8dispersion0 the black bands and hyperbolas may be rainbow-hued1 as with titanite4 ,footnote #ge3 ,the interference figure1 perpendicular to the obtuse bisectrix1 would be of the same type with a larger axial angle4 ,ordinarily this figure would not come within the limits of the field of view of the microscope4 ,confusion may arise1 however1 but in a section perpendicular to the acute bisectrix the cross dissolves more slowly into the hyperbolas than in the case of a section perpendicular to the obtuse bisectrix4 ,at times it may be necessary to measure the axial angle to be sure4 ,when1 however1 the mineral is known1 the section perpendicular to the acute bisectrix can be recognized1 because if the mineral is optically positive the trace of the axial plane is parallel to a and if negative parallel to c4 ,footnote #gf3 ,for construction of quartz wedge1 see p4 #cd4 ,footnote #gg3 ,the wedge can be introduced in either of the several ways described for the introduction of the test-plates on p4 #cc4 ,footnote #gh3 ,for methods of measuring the axial angle1 see ,methods of ,petrographic-,microscopic ,research1 ,f4 ,e4 ,wright1 #aiaa1 p4 #adg4 ,for convenience in many cases only #b;,e is recorded1 as then an indication is given as to whether the axial angle is visible with an ordinary microscope 7arranged for observation with convergent light for interference figures74 ,if #b;,e is very large the axial angle can only be observed by covering the section with some transparent1 strongly refracting fluid4 ,for the ,seibert microscope with objective ,v the limit for good results is about #b;,e #ij-ajj4 ,footnote #gi3 ,for dispersion1 etc41 see ,a4 ,j4 ,moses' ,characters of ,crystals1 p4 #adj4 ,footnote #hj3 ,the system of crystallization of leucite has been the subject of much discussion4 ,its habit is isometric4 ,the consensus of opinion seems to be that leucite crystallizes in the isometric system1 but that the isometric molecular arrangement1 at least of the larger crystals1 cannot exist for the temperature and pressure at the earth's surface4 ,hence molecular displacement takes place1 giving rise to a more or less complicated apparent twinning1 and optical anomalies are noticed4 ,the isotropic character returns if the section is heated to #ejj ,c4 ,iddings' ,rock ,minerals1 p4 #bdi1 #aiaa4 ,footnote #ha3 ,c4 ,w4 ,knight1 ,canad4 ,rec4 of ,sci41 ,,ix1 ,no4 #e4 #bfe4 ,footnote #hb3 ,the interference colors of all minerals here recorded are those given by sections #j4jc mm4 in thickness 7very thin sections74 ,footnote #hc3 ,shown by ,mgge1 ,joly1 etc41 to be caused by radiations emanating from ,u1 ,th1 ,r1 etc4 ,iddings' ,rock ,minerals1 #aiaa1 p4 #ahi4 ,footnote #hd3 ,iddings' ,rock ,minerals1 #aiaa1 p4 #cib4 ,footnote #he3 ,see page #ca4 ,footnote #hf3 ,r4 ,d4 ,irving1 ,am4 ,jour4 ,sci41 ,june1 #ahhc4 ,footnote #hg3 ,granites of the central ,alps1 where the calcite crystals are intergrown with quartz4 ,footnote #hh3 ,the minerals ,nephelite1 leucite1 sodalite 7haynite and noselite7 and melilite are often grouped together under the name 8feldspathoides02 on account of their relation in rocks being equivalent to that of the feldspars4 ,footnote #hi3 ,oriented in conformity to the intergrowth with augite1 etc41 7#jaj7 and 7#ajj7 are reversed4 ,footnote #ij3 ,for other alteration processes1 see ,iddings' ,rock ,minerals1 p4 #chj4 ,footnote #ia3 ,may be difficult to determine in the case of prism zone sections1 showing large extinction angles4 ,footnote #ib3 ,see p4 #hh4 ,footnote #ic3 ,iddings' ,rock ,minerals1 p4 #def1 #aiaa4 ,footnote #id3 ,depending on whether the axial plane is parallel or at right angles to the clino pinacoid 7#jaj7 7the plane of symmetry71 we have micas of the second 7biotite7 or first order 7muscovite74 ,footnote #ie3 ,the distinction between zoisite and 7essentially orthorhombic1 but may be composite triclinic twins7 and clinozoisite 7monoclinic close to orthorhombic7 depends on differences in position of plane of optic axes2 axial figures shown by cleavage plates2 dispersion2 anomalous interference colors2 etc4 ,see ,weinschenk's ,die ,gesteinbildenden ,mineralien1 p4 #hc4 #aija4 ,footnote #if3 ,test not easily made on account of the very high order interference colors1 resulting from the strong double refraction4 ,footnote #ig3 ,see under quartz1 p4 #fc4 ,footnote #ih3 ,iddings' ,rock ,minerals1 p4 #bjh1 #aiaa4 ,footnote #ii3 ,on account of the weak double refraction the interference figures are not very sharp or well defined in thin sections4 ,in most cases only the black hyperbolas are seen1 without any colored curves4 ,footnote #ajj3 ,by heating feldspar crystals the axial angle decreases to #j and then increases in the plane of symmetry 7at right angles to its former position74 ,on cooling the axial angle returns to its former position if the temperature has not exceeded #ejj ,c4 ,if the temperature has been #fjj-ajjj ,c4 for some time the axial angle will not return to its former position4 ,this fact may give some clew as to the temperature at which the feldspar crystals formed4 ,footnote #aja3 ,this change to kaolin or clay in granite is called by ,dolomieu 8,la maladie du granit40 ,footnote #ajb3 ,see p4 #ca4 ,footnote #ajc3 ,hatch's ,introduction to the ,study of ,petrology1 p4 #cc4 ,footnote #ajd3 ,j4 ,w4 ,judd1 ,geol4 ,mag4 #c1 ,vol4 ,,vi1 p4 #bdc1 #ahhi4 ,footnote #aje3 ,the plagioclases have rather a complex composition2 but may be regarded as forming a series from the composition ,na,al,si#c,o#h7;,ab7 to the composition ,ca,al#b,si#b,o#h7;,an71 consisting for the most part of isomorphous mixtures of these types1 with some replacement by ,k,al,si#c,o#h4 ,the compositions of only a few of the common plagioclases are given above4 ,footnote #ajf3 ,the lath-shaped feldspars1 moulding the augite1 give to diabases the so-called 8ophitic0 structure1 ,fig4 #ab4 ,the peculiarity of this structure is that the feldspars crystallized before the augite1 which is contrary to the usual order of formation4 ,footnote #ajg3 ,for the positions of the optic axes1 bisectrices1 etc41 relative to the cleavage plates of the different plagioclases1 see ,iddings' ,rock ,minerals1 p4 #bbb1 #aiaa4 ,footnote #ajh3 ,die ,gesteinsbildenden ,mineralien 7with tables71 ,e4 ,weinschenk1 ,freiburg1 #aija4 tude sur la ,dtermination des ,feldspaths dans les ,plaques ,minces1 ,michel ,lvy1 ,paris1 #ahid2 and ,the ,determination of the ,feldspars1 ,n4 ,h4 ,winchell1 ,am4 ,geol41 ,vol4 ,,xxi1 ,no4 #a1 #ahih4 ,iddings' ,rock ,minerals1 ,wiley ,sons1 #aiaa4 tude sur la ,dtermination des ,feldspaths 7troisime fascicule71 ,michel ,lvy1 ,paris1 #aijd4 ,footnote #aji3 ,glass models of the feldspars 7size #bj #aj cm47 by ,f4 ,krantz1 ,bonn4 ,diagrams1 showing optical orientation in the plagioclases1 in ,die ,gesteinsbildenden ,mineralien1 ,e4 ,weinschenk1 p4 #acc1 #aija4 ,footnote #aaj3 ,for this method of investigation little cleavage flakes or plates can often be obtained from the crushed mineral1 but1 on account of ,albite twinning1 plates are more apt to be obtained parallel to the twinning plane than to the best basal cleavage4 ,if a fragment with only one cleavage surface is obtained1 it must be cemented to a glass by this surface and ground down to a thin section with parallel sides4 ,footnote #aaa3 ,this test is only possible when suitable sections of the given feldspar in the rock section can be found4 ,the method1 however1 can be used with great accuracy with the aid of some form of apparatus for properly orienting the section4 ,see 8,klein's ,apparatus for the ,orientation of ,thin ,sections10 ,sitzungsber4 ,berlin4 ,akad41 #ahie1 #aaea2 7also in ,n4 ,y4 ,acad4 ,sci41 ,vol4 ,,xvi1 p4 #ea1 #ahig72 and ,von ,federov's 8,universal ,table10 ,zeit4 fr ,kryst1 etc41 ,vol4 ,,xxv41 p4 #cea4 ,footnote #aab3 ,in the determination of feldspar microlites it is well to remember the following facts3 8,microcline is rarely1 or never1 seen in the condition of microlites1 while the associations of labradorite and albite are so different that there is little danger of confounding them4 ,labradorite is the commonest product of the consolidation of the basic eruptives1 and albite almost invariably results from metamorphism1 frequently from the contact of igneous rocks on the calcareous clastics40 ,n4 ,h4 ,winchell1 ,determination of the ,feldspars1 ,am4 ,geol41 ,vol4 ,,xxi1 ,no4 #a1 p4 #cc1 #ahih4 ,footnote #aac3 ,these methods 7both #b and #c7 are often not applicable on account of the tendency of the crystals in an effusive rock to parallel orientation1 which may be so marked that the rock section does not show any favorable sections of the plagioclase4 ,footnote #aad3 ,these extinction angles1 as well as those previously given1 are those of only a few type feldspars of definite composition4 ,as the composition varies through a long series1 so the extinction angle changes1 one being a function of the other4 ,for a complete list of compositions and related extinction angles1 see ,iddings' ,rock ,minerals and ,lvy ,lacroix's ,les ,minraux des ,roches4 ,footnote #aae3 ,iddings' ,rock ,minerals1 p4 #bbh1 ,wiley ,sons1 #aiaa4 tude sur la ,dtermination des ,feldspaths 7troisime fascicule71 ,michel ,lvy1 ,paris1 #aijd4 ,footnote #aaf3 ,in quartz is the refractive index of the ray with vibration direction a that is the direction of vibration of the faster ray 7the ordinary ray74 ,hence is direction a and c4 ,in the feldspars2 a1 c and b4 ,footnote #aag3 ,am4 ,jour4 ,sci41 ,may1 #aijf4 ,this method is specially useful in detecting presence of orthoclase1 when plagioclase is the dominant feldspar4 ,footnote #aah3 ,these tests are only possible on pure and fresh material4 ,the specific gravity increases with the ,ca 7albite #b4fb1 anorthite #b4ge74 ,footnote #aai3 ,in clear unstriated granules1 which may be distinguished from quartz by biaxial interference figure in convergent light4 ,footnote #abj3 ,the tendency of labradorite in gabbros to twinning1 after both ,albite and ,pericline laws1 is to be noted4 ,footnote #aba3 ,werveke's 7,n4 ,j4 ,b41 #ahhc1 ,,ii1 #ig7 theory is that a twin lamination may be caused by the forces producing mechanical deformations1 as movement in the magma and mountain making pressure4 ,such lamell are characterized by the fact that their extent and course seem to depend on fracture lines in the crystal4 ,footnote #abb3 ,the derivation from pyroxene and amphibole appears to be doubtful1 see ,weinschenk's ,gesteinsbildenden ,mineralien1 #aija1 p4 #aba4 ,footnote #abc3 ,harker's ,petrology for ,students1 p4 #fc1 #ahie4 ,footnote #abd3 ,sections can be obtained from ,voight ,hochgesang1 ,gttingen2 ,c4 ,marchand1 rue ,censier1 #after1 ,paris2 ,w4 ,h4 ,tomlinson1 ,swarthmore1 ,pa41 and ,g4 ,d4 ,julien1 #c ,webster ,terrace1 ,new ,rochelle1 ,n4 ,y4 ,footnote #abe3 ,made by ,g4 ,d4 ,julien1 #c ,webster ,terrace1 ,new ,rochelle1 ,n4 ,y4 ,price3 ,power lathes 7complete71 #aej4jj up2 ,foot-treadle lathes1 #ij4jj up4 ,footnote #abf3 ,a ,crocker-,wheeler motor of to ,h4 ,p4 will be large enough for an ordinary laboratory machine4 ,footnote #abg3 ,made by ,voigt and ,hochgesang1 ,gttingen4 ,price1 #ae4jj4 ,footnote #abh3 ,saws charged with diamond dust can be obtained from ,elisha ,t4 ,jenks1 ,middleboro1 ,mass4 ,price in #aiaj1 #a4ce per diametrical inch4 ,foreign saws #f and #h diam4 can be bought from ,g4 ,d4 ,julien1 ,new ,rochelle1 ,n4 ,y4 ,footnote #abi3 ,school of ,mines ,quarterly1 ,vol4 ,,xi1 p4 #cb4 ,footnote #acj3 8,half a pound or less of ordinary shellac is melted in a flat-bottomed open vessel over a ,bunsen burner4 ,then an equal quantity of ,venice turpentine is carefully added under constant stirring4 ,the mass should be allowed to boil for about ten minutes1 during which the stirring is continued4 ,then small quantities are poured in separate heaps on an iron plate or other cold surface and rolled into sticks about seven inches long and half an inch thick40 ,footnote #aca3 ,square or standard #bf #df mm4 glass slides are recommended instead of the oblong slides 7#a #c inches71 which are very apt to project beyond the edges of the stage and be struck by the fingers while rotating the stage4 ,footnote #acb3 ,reference can be made to the publications on this subject by3 ,borichy1 ,behrens 7,behren's translation by ,judd71 ,haushofer1 ,huysse1 ,klment1 ,streng1 ,rnard1 etc4 ,footnote #acc3 ,in case an acid is to be used which would attack glass1 the cover-glass can be replaced by a thin perforated disk of platinum4 ,footnote #acd3 ,the bases in solution can be determined by different methods of analysis1 for which the student is referred to more elaborate works on this subject4 ,in some cases it may be very advantageous to treat the section with acid1 to remove certain soluble constituents1 when other minerals not distinctly seen at first may be made more apparent4 ,footnote #ace3 ,the gas may be ,h#b,s from a soluble sulphide1 in which case the solution containing the bubbles will color filter paper moistened with lead water4 ,footnote #acf3 ,geol4 and ,nat4 ,history ,survey of ,minn41 ,,xix41 ,ann4 ,rept41 p4 #db2 also ,a4 ,j4 ,moses1 ,characters of ,crystals1 p4 #adg4 ,footnote #acg3 ,the symmetry of the etched figures would1 of course1 be related to the system of crystallization4 ,footnote #ach3 ,a4 ,j4 ,moses1 ,characters of ,crystals1 ,chap4 ,,xvi4 ,footnote #aci3 ,when it is desired to preserve the section and at the same time to study the surface covered with a film of air1 the edges alone of the cover-glass should be cemented4 ,footnote #adj3 ,geol4 and ,nat4 ,hist4 ,survey of ,minn41 ,,xix1 ,ann4 ,rept41 p4 #ej4 ,thin sections of #b-c sq4 mm4 area are of a convenient size1 and they should be subjected to a red heat for 7#a7-c minutes4 ,too long a continuance of heat may render the sections too dark or lessen their transparency or produce melting4 ,footnote #ada3 ,this test may vary1 in many cases graphite not being consumed even after long heating4 ,footnote #adb3 ,the isolation of material for investigation is of more interest for the lithologist or chemist than for the student of optical mineralogy2 therefore only a very brief outline of some of the methods employed will be given4 ,footnote #adc3 ,the grains passing through different meshes are investigated microscopically to ascertain which size grains are homogeneous2 the rest of the sample should then be reduced to grains of this size4 ,footnote #add3 ,further reference can be made to ,iddings' ,rock ,minerals1 pp4 #be and #ie1 #aiaa4 ,a convenient form of apparatus is also described in ,school of ,mines ,quarterly1 ,vol4 ,x41 p4 #bhd1 #ahhi4 ,footnote #ade3 ,s4 ,l4 ,penfield1 ,am4 ,jour4 ,sci41 ,vol4 ,l41 p4 #ddf1 #ahie4 ,in this article a convenient form of separating apparatus is also described4 ,footnote #adf3 ,a convenient list of minerals1 arranged in the order in which they would be attracted by increasing the force of the electro-magnet1 is given in ,weinschenk's ,tabellen4 ,footnote #adg3 ,for micro-chemical work the reagents should be applied in very small drops1 which spread out on glass to discs #b mm4 in diameter4 ,for manipulating the reagents use platinum wires1 #j4e mm4 in thickness4 ,footnote #adh3 ,elemente einer neuen chemisch-mikroskopischen mineral- und ,gesteins-analyse1 ,pragg1 #ahgg4 ,translation of above by ,winchell in ,geol4 and ,nat4 ,hist4 ,survey of ,minn41 ,vol4 ,,xix41 ,ann4 ,report1 #ahij4 ,footnote #adi3 ,the strength of the solution should be about #c2 for if too weak many minerals do not give satisfactory results1 and if too strong a very large number of fluosilicate crystals are formed together with the separation of much silica1 thus making it impossible to carefully differentiate the crystals with a microscope4 ,footnote #aej3 ,as most of the rock-forming minerals that would be investigated are silicates1 the hydrofluosilicic solutions can also be obtained by treatment with ,h,fl4 ,footnote #aea3 ,after ,lvy and ,lacroix4 ,footnote #aeb3 ,naturkunde1 ,amsterdam1 #b1 ,vol4 ,,xvii41 #ahha4 ,chemical ,news1 ,vol4 ,,lxiii41 ,no4 #afdg1 ,june #ahia1 et seq4 ,micro-chemical ,analysis1 ,behrens 7,judd71 ,macmillan ,co41 ,london1 #ahid4 ,footnote #aec3 ,after ,lvy and ,lacroix4 ,footnote #aed3 ,after ,lvy and ,lacroix4 ,footnote #aee3 ,compiled from ,weinschenk's ,tables1 as revised in ,petrographic ,methods by ,weinschenk-,clark1 #aiab4 ,footnote #aef3 ,pirsson and ,robinson1 ,am4 ,jour4 ,sci41 iv1 ,vol4 ,x1 ,oct41 #aijj4 ,footnote #aeg3 ,harker1 ,petrology for ,students1 p4 #bh1 #ahie4 ,footnote #aeh3 ,have the convergent lens or condenser lowered and the analyzer out during this test4 ,footnote #aei3 ,the minerals are grouped according to the maximum interference colors given by sections of the thickness of #j4jc mm4 ,footnote #afj3 ,in the case of very small crystals1 the centering must be accurate and a cover with a small hole in the center1 should be placed over the top of the tube4 ,in this way the eye is brought directly over the axis of the microscope and figures can be observed from very minute crystals4 ---------------------------------------- -------------------------------- ,,transcriber',s ,,notes ,page ,changed from ,changed to #hj occurs as an alternation product occurs as an alteration product of anhydrite4 ,gypsum is soluble of anhydrite4 ,gypsum is soluble in hydrochloric in hydrochloric #acf longer the higher the percentage longer the higher the percentage of calcium in the solution of sodium in the solution ,typos fixed2 non-standard spelling and dialect retained4 ,the author used the old chemical symbol 7,fl7 for ,fluorine4 ,corrected the ,errata with the exception of the last paragraph ,corrections for mean indices of refraction in ,scheme 7insert folder74 ,used numbers for footnotes1 placing them all at the end of the last chapter4 ,enclosed italics font in underscores4 ,enclosed bold font in lower half block4 ,the caret 77 serves as a superscript indicator1 applicable to individual characters 7like #b;d7 and even entire phrases 7like #ast74 ,subscripts are shown using an underscore 77 with curly braces 1 as in ,h#b,o4