,,bureau ,,of ,,economic ,,geology ,the ,university of ,texas ,austin1 ,texas ,peter ,t4 ,flawn1 ,director ,guidebook #f ,,texas ,,rocks ,,and ,,minerals ,an ,amateur's ,guide ,by ,,roselle ,m4 ,,girard ,sketches by ,bill ,m4 ,harris ,february #aifd ,second ,printing1 ,april #aigb ,third ,printing1 ,april #aigf ,fourth ,printing1 ,may #aigi ,contents ,page ,preface vii ,introduction #a ,earth's outer crust #b ,geologists #b ,time and rock units #b ,geologic map #f ,what are rocks and minerals8 #g ,chemical elements #g ,minerals #g ,rocks #h ,igneous rocks #i ,extrusive or volcanic igneous rocks #i ,intrusive igneous rocks #i ,sedimentary rocks #aj ,soils #aj ,sedimentary rock materials in broken fragments #aa ,sedimentary rock materials in solution #ab ,cementing materials and chemical sediments #ab ,sedimentary rocks formed by plants and animals #ab ,metamorphic rocks #ab ,static metamorphism #ac ,contact metamorphism #ac ,dynamic metamorphism #ad ,occurrence and properties of minerals #ad ,how minerals occur #ad ,crystalline minerals #ad ,crystals #ad ,imperfect crystals #ad ,amorphous minerals #ae ,some distinguishing properties of minerals #ae ,color #af ,luster #af ,transmission of light #af ,hardness #af ,streak or powder #ag ,cleavage #ag ,parting #ag ,fracture #ag ,specific gravity #ah ,effervescence in acid #ah ,some special occurrences of minerals #ah ,cave deposits #ah ,concretions #ai ,geodes #ai ,petrified wood #bj ,collecting rocks and minerals #bb ,rock and mineral identification charts #bd ,how to use the mineral identification charts #bd ,key to mineral identification charts #be ,mineral identification charts #bf ,how to use the rock identification charts #ci ,rock identification charts #dj ,descriptions of some ,texas rocks and minerals #dc ,anhydrite #dc ,asbestos #dc ,barite #dd ,basalt #de ,calcite #df ,cassiterite #dg ,celestite #dh ,cinnabar #di ,clay #ea ,copper minerals 7chalcocite1 chalcopyrite1 malachite1 azurite7 #eb ,dolomite #ed ,feldspar #ee ,fluorite #ef ,galena #eg ,garnet #eh ,gneiss #ei ,gold #ei ,granite #fa ,graphite #fb ,gypsum #fc ,halite #fe ,hematite #ff ,limestone #fh ,limonite #gj ,llanite #ga ,magnetite #gb ,manganese minerals 7braunite1 hollandite1 pyrolusite7 #gc ,marble #ge ,mica #gf ,obsidian and vitrophyre #gg ,opal #gh ,pegmatite #gi ,pyrite #hj ,quartz #ha ,quartzite #hd ,rhyolite #he ,sand and sandstone #he ,schist #hg ,serpentine #hg ,shale #hh ,silver minerals 7argentite1 cerargyrite1 native silver7 #hi ,sulfur #ij ,talc and soapstone #ic ,topaz #id ,tourmaline #id ,uranium minerals 7carnotite1 uranophane1 pitchblende7 #ie ,volcanic ash 7pumicite7 #ig ,composition1 hardness1 and specific gravity of some ,texas minerals #ii ,books about rocks and minerals #ajj ,nontechnical books for beginners #ajj ,textbooks and other reference books #ajj ,selected references on ,texas rocks and minerals #ajj ,glossary #ajb ,index #ajd ,illustrations ,page ,guadalupe ,peak and ,el ,capitan in the ,guadalupe ,mountains1 ,culberson ,county1 ,texas #a ,earth's outer crust #b ,geologic time scale #c ,generalized geologic map of ,texas #d-e ,a mineral is made up of chemical elements #g ,a rock is made up of minerals #h ,extrusive igneous rocks form at the earth's surface #i ,intrusive igneous rocks form beneath the earth's surface #aj ,soils develop from weathered rock and associated organic material #aa ,conglomerate from ,webb ,county1 ,texas #aa ,precipitated sediments lining a teakettle #ab ,contact metamorphism #ac ,a scalenohedron #ad ,barite specimen showing radial form #ae ,chalcedony showing botryoidal form #af ,transparent mineral #af ,streak plate #ag ,conchoidal fracture #ah ,stalactites and stalagmites in the ,caverns of ,sonora1 ,sutton ,county1 ,texas #ai ,calcite geode from ,travis ,county1 ,texas #bj ,petrified wood from ,texas ,gulf ,coastal ,plain #bj ,prospector's hammer #bb ,hand lens #bb ,physiographic outline map of ,texas #db ,massive anhydrite #dc ,amphibole asbestos from ,gillespie ,county1 ,texas #dd ,barite cleavage fragment from west ,texas #dd ,basalt from ,brewster ,county1 ,texas #de ,calcite has perfect rhombohedral cleavage #df ,calcite crystals 7dog-tooth spar7 from the ,terlingua area of ,brewster ,county1 ,texas #dg ,celestite cleavage fragment from ,lampasas ,county1 ,texas #dh ,cinnabar and calcite crystals from the ,terlingua area of ,brewster ,county1 ,texas #ej ,bentonite is used as a drilling-fluid additive #ea ,hazel copper-silver mine1 ,culberson ,county1 ,texas #ec ,dolomite rock from ,burnet ,county1 ,texas #ed ,feldspar cleavage fragment from ,llano ,county1 ,texas #ee ,microcline feldspar crystals from ,llano ,county1 ,texas #ef ,fluorite has octahedral cleavage #eg ,galena has perfect cubic cleavage #eg ,garnet crystal forms #eh ,gneiss from ,blanco ,county1 ,texas #ei ,placer gold in stream gravels #fj ,granite from ,gillespie ,county1 ,texas #fa ,texas ,state ,capitol building at ,austin is made of ,burnet ,county granite #fb ,graphite is used in pencil lead1 generator brushes1 and lubricants #fc ,selenite gypsum crystal from ,bastrop ,county1 ,texas #fd ,selenite gypsum rosettes from ,nolan ,county1 ,texas #fd ,fibrous gypsum from ,terlingua area1 ,brewster ,county1 ,texas #fe ,salt domes occur on the ,gulf ,coastal ,plain #ff ,specular hematite from ,carrizo ,mountains1 ,hudspeth ,county1 ,texas #fg ,limestone from ,travis ,county1 ,texas #fh ,limestone quarry at ,georgetown1 ,williamson ,county1 ,texas #fi ,limonite ore is changed to metallic iron in a blast furnace #ga ,metallic iron is made into steel in an open-hearth furnace #gb ,magnetite1 ,llano ,county1 ,texas #gc ,hollandite from ,jeff ,davis ,county1 ,texas #gd ,precambrian metamorphic marble from ,llano ,county1 ,texas #ge ,mica minerals have perfect cleavage in one direction #gf ,obsidian arrowheads #gg ,opalized wood from ,washington ,county1 ,texas #gh ,quartz-feldspar pegmatite from ,burnet ,county1 ,texas #gi ,pyrite veins in white marble from ,llano ,county1 ,texas #hj ,cubic crystals of pyrite #hj ,quartz crystal from ,burnet ,county1 ,texas #ha ,amethyst geode from the ,alpine area of ,brewster ,county1 ,texas #hb ,milky quartz from ,burnet ,county1 ,texas #hb ,smoky-quartz crystals from ,burnet ,county1 ,texas #hc ,polished agate from ,rio ,grande gravels of ,zapata ,county1 ,texas #hc ,jasper from ,uvalde ,county1 ,texas #hd ,sandstone from ,zavala ,county1 ,texas #hf ,prospector #hi ,sulfur is obtained by the ,frasch process #ib ,talc schist from the ,allamoore area of ,hudspeth ,county1 ,texas #ic ,topaz crystal from ,mason ,county1 ,texas #id ,black tourmaline crystals with milky quartz from ,llano ,county1 ,texas #ie ,a ,geiger counter is used to detect radioactivity #if ,,preface ,this booklet has been designed to serve as a brief1 simple guide that will be of help to school children1 amateur collectors1 and others who are just beginning to develop an interest in the rocks and minerals of ,texas4 ,it is a companion volume to ,texas ,fossils by ,william ,h4 ,matthews ,,iii published as ,guidebook ,no4 #b by the ,bureau of ,economic ,geology4 ,numerous present and former staff members of ,the ,university of ,texas contributed time and talents to the preparation of this book1 and their help is gratefully acknowledged3 ,peter ,t4 ,flawn1 ,director of the ,bureau of ,economic ,geology1 ,thomas ,e4 ,brown1 ,john ,w4 ,dietrich1 ,alan ,humphreys1 ,elbert ,a4 ,king1 ,jr41 ,peter ,u4 ,rodda1 and others1 including the late ,john ,t4 ,lonsdale1 made many helpful suggestions2 ,john ,s4 ,harris and ,miss ,josephine ,casey edited the manuscript2 ,cader ,a4 ,shelby prepared a number of the photographs2 ,bill ,m4 ,harris made the illustrative sketches under the direction of ,james ,w4 ,macon2 and ,cyril ,satorsky designed the cover4 ,texas ,rocks and ,minerals ,an ,amateur's ,guide ,roselle ,m4 ,girard ,,introduction ,texas has a great variety of rocks and minerals--some are common and others are not4 ,this book is designed to acquaint you with some of them and to tell you in a nontechnical way what they are like1 some of the places where they are found1 and how they are used4 ,although we do not know exactly how all of the rocks and minerals formed1 some of the ideas about their origin are mentioned4 ,if you would like to learn more about rocks and minerals in general1 the names of several reference books are listed on page #ajj4 ,in addition1 scientific reports that describe in detail many of the rocks and minerals of ,texas have been published by the ,bureau of ,economic ,geology of ,the ,university of ,texas1 the ,united ,states ,geological ,survey1 and other organizations4 ,a selected list of these reports is given on pages #ajj-aja4 ,rocks and minerals are familiar objects to all of us4 ,we pick up attractive or unusual pebbles for our collections1 we admire rocky mountain peaks1 we speak of the mineral resources of our ,state and ,nation4 ,rocks and minerals enter1 either directly or indirectly1 into our daily living4 ,from them come the soils in which grow the grains1 the fruits1 and the vegetables for our food1 the trees for our lumber1 and the flowers for our pleasure4 ,the iron1 copper1 lead1 gold1 silver1 and manganese1 the sulfur and salt1 the clays and building stones1 and the other metals and nonmetals that we require for our way of living were once a part of the earth's crust4 ,illustration3 ,texas' highest mountain is ,guadalupe ,peak1 right1 with an elevation of #h1gea feet4 ,el ,capitan1 left1 has an elevation of #h1jgh feet4 ,these peaks in the ,guadalupe ,mountains in ,culberson ,county consist largely of ,capitan reef limestone1 which formed during the ,permian ,period4 ,earth's ,outer ,crust ,rocks and minerals make up most of the outer layer or crust of our earth--the actual ground beneath our feet4 ,the crust is approximately #ah to #cj miles thick beneath the continents4 ,in general1 the outermost part consists of many layers of stratified rocks1 one above another4 ,the older rocks normally make up the bottom or the deeper layers1 and the younger rocks form the upper layers4 ,not all the layers are perfectly flat and parallel--some are lenticular 7lens-shaped71 some are tilted1 some are partly eroded away1 and some are present in one place and absent in another4 ,beneath the continents1 the layers of rock rest on ancient metamorphic rocks and on great masses of igneous rock such as granite4 ,these lower rocks are known as the basement4 ,illustration3 ,earth's outer crust 7thickness not drawn to scale74 ,over much of the land surface of the earth1 the outermost layer is made up of layers of rock ,on the continents1 the layers of rock rest on metamorphic rocks and on igneous rocks such as granite ,geologists ,those who study the earth's crust--its origin1 history1 rocks1 minerals1 fossils1 and structure--are known as geologists4 ,the geologists who are especially interested in a particular phase of geology1 as this science is called1 are given special names3 those who study fossils are called paleontologists2 those who study minerals are called mineralogists2 those who study rocks are called petrologists4 ,time and ,rock ,units ,the earth's crust is believed to be at least #c billion years old4 ,in order to deal with this vast stretch of time1 geologists have divided the billions of years into various time units and have given each unit a name4 ,the great divisions of geologic time1 called eras1 are ,early ,precambrian1 ,late ,precambrian1 ,paleozoic1 ,mesozoic1 and ,cenozoic4 ,these eras are divided into smaller units of time called periods1 and the periods are divided into epochs4 ,the xx time scale shows the geologic time divisions4 ,earliest geologic time is shown at the bottom of the scale2 most recent is shown at the top4 ,by examining and studying the different rocks and rock layers1 geologists try to discover in which unit of geologic time these rocks formed4 ,those rocks that formed during a period of geologic time are called a system of rocks2 those that formed during an epoch are called a series4 ,for example1 the ,cambrian ,system of rocks formed during the ,cambrian ,period2 the ,cretaceous ,system of rocks formed during the ,cretaceous ,period2 the ,tertiary ,system of rocks formed during the ,tertiary ,period4 ,we are now in the younger epoch 7called ,recent7 of the ,quaternary ,period of the ,cenozoic ,era4 ,the rocks that are forming now are the ,recent ,series of rocks4 ,illustration3 ,geologic time scale ,,era ,,period ,,epoch ,,cenozoic ,,quaternary 7lasted #j-a million years7 ,recent ,pleistocene ,,tertiary 7lasted #fb million years7 ,pliocene ,miocene ,oligocene ,eocene ,paleocene --#fc million years ago-- ,,mesozoic ,,cretaceous 7lasted #gb million years7 ,,jurassic 7lasted #df million years7 ,,triassic 7lasted #di million years7 --#bcj million years ago-- ,,paleozoic ,,permian 7lasted #ej million years7 ,,pennsylvanian 7lasted #cj million years7 ,,mississippian 7lasted #ce million years7 ,,devonian 7lasted #fj million years7 ,,silurian 7lasted #bj million years7 ,,ordovician 7lasted #ge million years7 ,,cambrian 7lasted #ajj8 million years7 --#fjj8 million years ago-- ,,late ,,precambrian ,,early ,,precambrian ,these time estimates are from the paper1 ,geologic ,time ,scale1 by ,j4 ,lawrence ,kulp1 published in ,science1 ,vol4 #acc1 ,no4 #cdei1 ,april #ad1 #aifa4 7,the time divisions are not drawn to scale7 ,illustration3 ,plate #aj4 ,,generalized ,,geologic ,,map ,,of ,,texas ,modified from ,geologic ,map of ,texas1 #aicc ,,explanation ,,cenozoic #a ,quaternary #b ,tertiary 7,oligocene1 ,miocene1 and ,pliocene7 #c ,tertiary 7,eocene7 #d ,volcanic 7extrusive7 igneous rocks ,,mesozoic #e ,upper ,cretaceous 7,gulf series7 #f ,lower ,cretaceous 7,comanche series7 #g ,jurassic #h ,triassic ,,paleozoic #i ,permian #aj ,mississippian and ,pennsylvanian #aa ,cambrian1 ,ordovician1 ,silurian1 ,devonian and undivided ,paleozoic #ab ,rocks 7,precambrian7 older than ,paleozoic #ac ,intrusive igneous rocks 7,precambrian1 ,mesozoic or ,cenozoic7 ,these rocks are found either at the surface or directly beneath the soils and subsoils which cover most of ,texas4 ,geologists also subdivide rocks into lesser units4 ,one of these1 called a group1 is made up of two or more formations4 ,a formation comprises rocks or strata 7layers of rock7 that are recognized and mapped as a unit4 ,some formations consist of layers of one particular type of rock1 such as limestone or shale4 ,formations are named after a nearby geographic locality1 and in some formation names1 the type of rock is included4 ,for example1 three of the ,texas geologic formations are called ,buda ,limestone1 ,del ,rio ,clay1 and ,eagle ,ford ,shale4 ,geologic ,map ,the geologic map 7pp4 #d-e7 shows the rocks that are found at the surface in ,texas4 ,some of these are extremely old4 ,some1 geologically speaking1 are very young4 ,,what ,,are ,,rocks ,,and ,,minerals8 ,although rocks and minerals are often mentioned together1 and to some people they have similar meanings1 geologists make a distinction between the two words4 ,in general1 rocks are made up of minerals1 and minerals are made up of chemical elements4 ,chemical ,elements ,the chemical elements include oxygen1 silicon1 calcium1 sulfur1 carbon1 gold1 silver1 and many others4 ,there are #ij naturally occurring elements4 ,each is made up of molecules that consist of only one kind of atom4 ,chemical elements may either be combined with each other or occur alone4 ,they are the building blocks of our world for they make up all the gases1 all the liquids1 all the minerals1 all the plant and animal life1 and all the other physical matter4 ,some of the chemical elements that occur in the rocks and minerals mentioned in this book are listed below4 ,aluminum ,al ,barium ,ba ,beryllium ,be ,boron ,b ,calcium ,ca ,carbon ,c ,cerium ,ce ,chlorine ,cl ,copper ,cu ,fluorine ,f ,gold ,au ,hydrogen ,h ,iron ,fe ,lead ,pb ,magnesium ,mg ,manganese ,mn ,mercury ,hg ,molybdenum ,mo ,oxygen ,o ,potassium ,k ,silicon ,si ,silver ,ag ,sodium ,na ,strontium ,sr ,sulfur ,s ,thorium ,th ,tin ,sn ,uranium ,u ,vanadium ,v ,yttrium ,y ,zinc ,zn ,zirconium ,zr ,we can compare the chemical elements to the letters of our alphabet4 ,the letters1 like the chemical elements1 are fundamental building blocks1 and they can be brought together in various combinations to form words4 ,minerals ,a mineral can be compared to a word of our language4 ,we combine letters to form a word1 and nature combines certain chemical elements to form each particular mineral4 ,for example1 calcite1 a mineral that is abundant in ,texas1 is always made up of the same proportions of the same three elements3 calcium1 carbon1 and oxygen4 ,illustration3 ,a mineral is made up of chemical elements4 ,the mineral calcite1 for example1 always consists of the same proportions of calcium1 carbon1 and oxygen4 ,each mineral has its own characteristic internal structure and other properties4 ,at ordinary temperatures1 nearly all the minerals are solids rather than gases or liquids4 7,water and mercury are the principal exceptions47 ,in addition1 minerals are inorganic rather than being composed of plant or animal matter4 ,when a single chemical element is found alone in nature as a solid1 it is considered to be a mineral1 too4 ,gold1 silver1 copper1 lead1 and sulfur are some of the chemical elements that can occur alone as solid minerals4 ,when they occur this way1 we refer to them as native silver1 native copper1 or native sulfur4 ,although the element mercury is a liquid rather than a solid at ordinary temperatures1 it too is a mineral when it occurs alone in nature4 ,it is then called native mercury4 ,rocks ,we have already compared the chemical elements to the alphabet and the minerals to words4 ,we can now go a step further and compare rocks to sentences4 ,we put words together to make sentences2 nature puts minerals together to make rocks4 ,a sentence does not have to be made up of a definite number of words1 nor does a rock have to be made up of a definite number of minerals4 ,some rocks1 such as granite1 may be composed of several minerals4 ,others1 such as dolomite and rock gypsum1 consist of only one mineral4 ,minerals do not lose their identities when they make up a rock4 ,instead1 they are merely associated together in varying proportions4 ,some rocks1 as we will find later1 instead of being composed of the minerals themselves1 are made up of fragments of earlier-formed rocks4 ,ordinarily1 we think of rocks as hard and solid substances1 such as limestone and granite1 but some geologists consider loose and uncemented materials1 such as sand1 gravel1 or volcanic ash1 to be rocks also4 ,the words sediments or deposits are often used to describe this uncemented or loose material4 ,rocks are commonly grouped1 according to how they formed1 into three great classes known as igneous1 metamorphic1 and sedimentary4 ,illustration3 ,a rock is made up of minerals4 ,the igneous rock granite1 for example1 consists chiefly of quartz and feldspar2 other minerals such as mica and hornblende are commonly present4 ,,igneous ,,rocks ,igneous rocks result from the cooling of hot1 molten rock material or magma4 ,magma that reaches the surface through volcanoes is called lava4 ,magma comes from deep within the earth and is made up of a mixture of molten mineral materials4 ,igneous rocks have been forming throughout the geologic past and are still forming today4 ,we can understand how they form when we look at pictures of hot1 molten lava flowing from volcanoes1 such as ,mauna ,loa in ,hawaii4 ,as lava cools1 it hardens into rock4 ,extrusive or ,volcanic ,igneous ,rocks ,the igneous rocks that form on the earth's surface are called extrusive or volcanic igneous rocks4 ,when magma flows to the surface1 it cools and hardens quickly4 ,the mineral grains that form during this fast cooling may be too small to be distinguished from each other4 ,some lava cools too quickly for minerals to crystallize--then the rock is volcanic glass4 ,illustration3 ,extrusive igneous rocks form at the earth's surface from lava that cools and hardens relatively quickly4 ,no volcanic igneous rocks are forming in ,texas now4 ,however1 during ,tertiary time1 in the ,big ,bend area and in other parts of the ,trans-,pecos country of west ,texas1 lava came to the surface and hardened4 7,the physiographic outline map1 p4 #db1 shows where these areas are located47 ,intrusive ,igneous ,rocks ,the cooling and hardening of hot1 molten magma also takes place below the earth's surface4 ,here1 the magma cools slowly to form rocks made up of mineral grains that are large enough to be readily visible4 ,these rocks are known as intrusive igneous rocks4 ,we know that they are present below the surface in ,texas because of wells drilled in many areas of the ,state4 ,in ,pecos ,county1 a well reached granite1 an intrusive igneous rock1 at a depth of #af1eaj feet4 ,other wells in ,texas have reached the granite basement rocks at much shallower depths4 ,but not all intrusive igneous rocks in ,texas are found underground4 ,in the ,trans-,pecos country of west ,texas1 in the ,balcones fault zone1 and in the ,llano uplift of central ,texas1 some are now seen at the surface4 ,they1 like all intrusive rocks1 were formed below the ground1 but earth's processes of uplift and erosion have gradually uncovered them4 ,illustration3 ,intrusive igneous rocks form from molten rock material 7magma7 that cools and hardens beneath the earth's surface4 ,,sedimentary ,,rocks ,sedimentary rocks are made up of sediments1 which are rock and mineral grains that have come from weathered rocks of all kinds4 ,rocks are weathered when water1 ice1 snow1 wind1 and other agents cause them either to dissolve1 as table salt does when put in water1 or to break apart1 as old pavement commonly does4 ,soils ,some of the broken-down rocks1 along with associated plant and animal matter1 develop into soils4 ,when you examine soil with a magnifying glass1 you may be able to see some of the small rock and mineral grains that still remain in it4 ,some soils have formed on top of the rocks from which they came1 and some have been moved in from another place4 ,illustration3 ,soils develop from weathered rock and associated organic material4 ,,soil ,,subsoil ,,weathered ,,rock ,,bedrock ,sedimentary ,rock ,materials in ,broken ,fragments ,water and wind not only weather the rocks and soils but also move the weathered materials 7the sediments7 and deposit them in other places4 ,whenever you see a dust or sand storm1 or a muddy creek or river1 you are observing the movement of sediments by wind and water to other land areas or to the sea4 ,the combination of weathering and movement is called erosion4 ,illustration3 ,conglomerate from ,webb ,county1 ,texas1 is composed of rounded gravel that has been cemented together4 ,some of the rock fragments carried by water are still fairly large when they reach their destinations4 ,on the basis of size1 they are called boulders1 cobbles1 pebbles1 and granules4 ,loose deposits of these larger-size sediments make up what is known as gravel4 ,nature cements gravels together to form rocks such as conglomerates 7made up of rounded gravel7 and breccias 7made up of sharp-cornered gravel74 ,the finer sediments are called sand1 silt1 mud1 and clay4 ,when cemented1 the sand grains become sandstones1 the silt particles become siltstones1 and the mud and clay particles become shale4 ,the sedimentary rocks that are made up of these rock fragments are called clastic or fragmental rocks4 ,sedimentary ,rock ,materials in ,solution ,as they are weathered1 some rocks dissolve and go into solution4 ,for example1 a number of the ,texas creeks and rivers carry calcium carbonate in solution because they flow through areas where limestone rocks1 which consist mostly of calcium carbonate1 are being weathered4 7,water that contains a large amount of dissolved rock material is called hard water47 ,cementing materials and chemical sediments4-- ,some of the waters containing dissolved rock material seep through loose sediments where the dissolved material may come out of solution and form a cement1 which binds the sediments together4 ,for example1 when loose sand sediments are cemented1 they form sandstone4 ,three of the most common cements are iron oxide1 calcium carbonate1 and silicon dioxide1 although a number of other materials also serve as cements4 ,dissolved rock materials come out of solution not only to serve as cementing agents but to form the chief mineral of some sedimentary rocks as well4 ,sedimentary rocks of this kind form mostly in lakes and seas into which much dissolved material is carried by rivers4 ,when the dissolved material comes out of solution1 it is said to be precipitated and the mineral sediments it forms are the chemical sediments4 ,some limestones originate this way4 ,you can see examples of precipitated materials by noting the crust-like deposits that form inside some water pipes and teakettles1 as dissolved material in the water comes out of solution4 ,illustration3 ,precipitated sediments are commonly observed lining a teakettle4 ,sedimentary rocks formed by plants and animals4-- ,the dissolved rock material can come out of solution in another way4 ,some plants and animals are able to take dissolved calcium carbonate out of the sea water and use it to build their shells and other structures4 ,some of these organisms1 such as corals and algae1 can grow upward from the sea floor in large groups to form reefs that later become reef limestones4 ,other limestones are made up of the remains of plants and animals that collect on the sea floor and become cemented together4 ,,metamorphic ,,rocks ,metamorphic rocks come from earlier-formed rocks that have undergone a change or a metamorphosis4 ,all igneous and sedimentary rocks1 and earlier-formed metamorphic rocks too1 can be changed1 without being moved to some other place1 into new and different rocks4 ,as they are changed1 they may become harder1 new minerals may form1 and they may look entirely different4 ,for example1 granite1 an igneous rock1 can be changed into the metamorphic rock known as gneiss2 limestone1 a sedimentary rock1 can be changed into marble2 shale1 a sedimentary rock1 can be changed into slate4 ,these changes occur because the earth is a big and complex chemical system4 ,the agents that bring about these changes1 which always occur below the surface of the earth1 are heat1 pressure1 and fluids--both liquids and gases4 ,several different kinds of change or metamorphism can take place4 ,static ,metamorphism ,some of the changes occur because the rocks are at great depths4 ,as more and more younger rocks are deposited on top of them1 the older rocks become deeply buried4 ,the great thicknesses of younger rocks are heavy1 and they squeeze and press down on the rocks beneath them4 ,the deeply buried rocks are also hotter than surface rocks4 ,in general1 the temperature increases about #a ,fahrenheit for each #ej feet of depth below the surface4 ,the change of deeply buried rocks into new rocks by pressure and heat is known as static metamorphism4 ,contact ,metamorphism ,another method of change or metamorphism involves molten igneous rock material4 ,when hot magma moves up through rocks1 it not only heats and pushes them1 but it also may soak them with liquids and gases1 causing the nearby rocks to change into new rocks1 by a process called contact metamorphism4 ,illustration3 ,some rocks are altered by heat and fluids when they are invaded by hot magma in a process called contact metamorphism4 ,,unaltered ,,rock ,,metamorphic ,,rock ,,magma ,dynamic ,metamorphism ,still another rock-changing process is one that is associated with mountain building4 ,when mountains are formed1 heat and great pressures develop deep within the earth's crust4 ,the flat layers of rock are then slowly pushed and squeezed so that they bend up into arches1 fracture1 or slide over each other4 ,these forces cause great changes in the rocks in widespread areas4 ,this process of change is known as dynamic metamorphism4 ,occurrence and ,properties of ,minerals ,,how ,,minerals ,,occur ,rocks are made up of minerals4 ,in addition1 minerals are associated with rocks in other ways4 ,for example1 minerals fill or coat cracks and cavities that have developed in some of the rocks4 ,minerals are either crystalline or amorphous4 ,crystalline ,minerals ,most minerals are crystalline4 ,in crystalline minerals1 combinations of atoms are arranged in ordered patterns1 which are repeated over and over4 ,this orderly internal structure of atoms is a characteristic of each crystalline mineral1 as mineralogists are able to determine by using ,x-rays and special microscopes4 ,crystals4-- ,when a mineral occurs as a well-formed individual crystal1 it has a definite1 precise shape4 ,the kind of crystal shape it has depends on its own type of crystalline internal structure4 ,a well-formed crystal has smooth1 flat1 outer surfaces called crystal faces1 which are arranged together to form prisms1 cubes1 pyramids1 and many other geometric shapes4 ,for example1 quartz1 a common ,texas mineral1 is commonly found as a six-sided1 prism-shaped crystal that is topped by pyramid-like forms4 ,pyrite1 another common mineral1 occurs as cube-shaped crystals4 ,we can identify some minerals more readily by learning to recognize their crystal shapes4 ,illustration3 ,a scalenohedron1 one of the many crystal forms of calcite4 ,imperfect crystals4-- ,a crystalline mineral commonly forms under conditions that do not permit it to become a well-shaped crystal4 ,although the mineral may show a few crystal faces1 it does not have a complete crystal shape and so is described as massive1 or is said to occur in masses4 ,some of the minerals that make up rocks occur as crystalline masses4 ,for example1 calcite is a crystalline mineral that occurs in the metamorphic rock marble without its normal crystal shape4 ,many crystalline minerals occur as incomplete and imperfect crystals that are grouped together in various arrangements4 ,if these incomplete crystals are arranged around a common center like the spokes of a wheel1 they are said to be radial or radiated4 ,if the groups of incomplete crystals look like bundles of strings or fibers1 they are described as fibrous4 ,if they are in rounded masses that resemble bunches of grapes1 they are called botryoidal4 ,if they look like fish scales1 they are described as scaly4 ,some crystalline minerals are made up of tiny grains that are grouped together like the grains in a lump of sugar4 ,a mineral occurring in this way is described as granular4 ,more descriptions of crystalline minerals are found in the section on ,texas rocks and minerals 7pp4 #dc-ih74 ,illustration3 ,barite specimen showing radial form4 ,amorphous ,minerals ,an amorphous mineral1 unlike a crystalline mineral1 does not have a definite1 orderly arrangement of its atoms4 ,because of this lack of internal structure1 the mineral occurs in masses that have no regular geometric shapes1 and it has no crystal form of its own4 ,only a few minerals are amorphous4 ,,some ,,distinguishing ,,properties ,,of ,,minerals ,we use our senses of sight1 hearing1 smell1 touch1 and taste to become aware of the world around us4 ,for example1 we recognize a flower by noting its color1 its fragrance1 and the texture1 shape1 and arrangement of its petals4 ,these are some of its characteristic properties4 ,a mineral also has distinguishing properties1 among them color1 luster1 and hardness1 which help us identify it4 ,some minerals have a single outstanding property1 such as the magnetism of magnetite1 that makes them easier to recognize4 ,but to identify most minerals1 we need to determine not just one1 but several properties4 ,illustration3 ,chalcedony showing botryoidal form4 ,color ,color is one of the properties we notice first4 ,the color of some minerals is always the same1 and it helps us to identify them4 ,but it is not a dependable property to use in identifying all minerals1 because some contain impurities that change or hide the real color4 ,luster ,the luster is the way the surface of a mineral reflects light4 ,the luster of a mineral may be nonmetallic1 submetallic1 or metallic4 ,mineral metals such as gold1 silver1 galena1 and pyrite have a metallic luster4 ,a few minerals have a luster that is almost1 but not quite metallic--their luster is submetallic4 ,a mineral with a nonmetallic luster may look vitreous 7glassy71 silky1 resinous 7like resin71 greasy1 earthy 7dull71 pearly1 or adamantine 7brilliant74 ,transmission of ,light ,some minerals allow light to pass through them2 others do not4 ,a mineral is transparent if you can see both light and objects through it1 as through clear glass4 ,if you can see only light1 but no objects1 as through frosted glass1 the mineral is translucent4 ,when you hold an opaque mineral up to the light1 it looks dark4 ,no light at all comes through it1 even through the thin edges4 ,illustration3 ,transparent mineral4 ,hardness ,some minerals are soft and can be scratched easily4 ,others1 which are harder1 are resistant to scratching4 ,to measure a mineral's hardness1 we try to find out which substances will scratch it and which substances will not scratch it4 ,to do this in a general way1 several ordinary objects--such as a fingernail1 a copper penny1 a pocket knife1 a piece of window glass1 and a steel file--can be used4 ,for a more exact way of testing hardness1 we can use ten minerals that make up what is known as ,mohs scale4 ,each mineral in this scale has a different hardness1 and each one has been given a number that represents its hardness4 ,for example1 talc1 the softest mineral in this scale1 is given a hardness of #a4 ,gypsum1 the next softest mineral in the scale1 has a hardness of #b4 ,diamond1 the hardest mineral known1 is given the top hardness of #aj in this scale4 ,these ten minerals are listed below4 ,alongside them are five common objects with their hardnesses4 #a--,talc #b--;,gypsum ,fingernail--slightly over #b #c--;,calcite ,copper penny--about #c #d--;,fluorite #e--;,apatite ,pocket knife--slightly over #e #f--,orthoclase ,window glass--#e #g--,quartz ,steel file--about #f #h--,topaz #i--;,corundum #aj--;,diamond ,suppose1 for example1 that a mineral can be scratched by fluorite1 which has a hardness of #d on ,mohs scale1 but cannot be scratched by calcite1 which has a hardness of #c4 ,we then know that this mineral is softer than fluorite1 but harder than calcite2 therefore1 it has a hardness of about #c4 ,in the same way1 if a mineral can be scratched by a pocket knife1 which is slightly more than #e in hardness1 but not by a copper penny1 which has a hardness of about #c1 we know then that its hardness is between #c and #e4 ,streak or ,powder ,the streak is the mark1 made of fine powder1 that a mineral leaves as you rub it across a streak plate4 ,a streak plate is a flat piece of white tile or porcelain that has a dull1 unglazed surface4 ,the streak plate is about as hard as quartz1 which is #g on ,mohs scale1 and you will not be able to use it for minerals that have a greater hardness4 ,for these1 you can obtain the powder by scratching the mineral or by crushing a small piece of it4 ,illustration3 ,a streak plate is used to determine the color of the streak or powder of a mineral4 ,the color of the streak or powder is extremely helpful in identifying some minerals4 ,for example1 hematite is a mineral that may be any one of several different colors1 but its streak or powder is always reddish brown4 ,cleavage ,as they break1 some crystalline minerals always split along a smooth1 flat surface4 ,this property is known as cleavage4 ,some cleavages are smooth and perfect2 others are not so perfect4 ,the cleavage surfaces1 because of the mineral's crystalline internal structure1 are parallel to possible crystal faces1 even though the mineral itself may occur as a crystalline mass without a perfect crystal shape4 ,some minerals will cleave in only one direction2 some1 in several directions4 ,for example1 galena1 a mineral found in ,texas1 has perfect cubic cleavage4 ,it cleaves in three directions that are at right angles to each other4 ,these cleavage directions are parallel to possible cubic crystal faces1 and some of the cleavage fragments are cubes4 ,parting ,a few minerals sometimes show a kind of false cleavage known as parting4 ,parting1 unlike cleavage1 is not constant and does not occur in every specimen of a particular mineral4 ,for this reason1 it is not a very dependable means of identification4 ,fracture ,minerals also break in another way4 ,when the break is in a different direction from that of the cleavage or parting1 it is known as the fracture4 ,a fracture is called conchoidal if the mineral's broken surface is curved like the inside of a spoon or shell4 ,thick pieces of glass break with this conchoidal fracture4 ,a fracture is described as hackly if the broken surface has sharp1 jagged edges2 as even1 if the surface is generally flat2 and as uneven1 if it is rough and not flat4 ,if the mineral breaks into splinters1 its fracture is called splintery4 ,illustration3 ,conchoidal fracture4 ,specific ,gravity ,the specific gravity is a measure of whether a mineral is heavy or light4 ,it is a comparison of the weight of a piece of the mineral with the weight of an equal volume of water4 ,the mineral quartz1 for example1 has a specific gravity of #b4fe4 ,this means that a piece of quartz is a little more than #b times as heavy as an equal volume of water4 ,accurate measurements of specific gravity can be made in a laboratory4 ,you can1 however1 learn to estimate specific gravities just by lifting various minerals and judging whether they are heavy or light4 ,effervescence in ,acid ,this is a property that depends on the chemical composition of the mineral4 ,carbonate minerals1 which contain 7in addition to at least one other element7 three parts of oxygen and one part of carbon1 can be tested with dilute hydrochloric acid4 ,when a drop or two of this acid is put on a carbonate mineral such as calcite 7calcium carbonate1 ,ca,c,o71 the acid begins to bubble and fizz4 ,the fizzing or effervescence is caused by the carbon dioxide gas that is formed when the acid and mineral come in contact with each other4 ,this test is also helpful in identifying rocks1 such as limestone and marble1 that contain carbonate minerals4 ,,some ,,special ,,occurrences ,,of ,,minerals ,cave ,deposits ,beautiful mineral deposits occur in some natural caves4 ,deposits that look like icicles1 called stalactites1 are found hanging from the ceiling of a cave4 ,other deposits1 stalagmites1 are like the stalactites except that they jut upward from the floor4 ,columns are formed from stalactites and stalagmites that have joined together4 ,in addition1 some caves contain sheet-like deposits that are spread along the ceiling1 floor1 and walls4 ,these deposits are called flowstone4 ,calcite is one of the minerals that commonly form cave deposits4 ,just a few of the caves in ,texas contain these deposits4 ,they occur mostly in the limestone rocks that are south and southwest of the ,llano uplift area of central ,texas4 ,some of the commercial caves that contain good examples of calcite deposits are located near ,boerne in ,kendall ,county and near ,sonora in ,sutton ,county4 ,calcite deposits also occur in ,longhorn ,cavern1 a large cave located in the ,longhorn ,cavern ,state ,park of ,burnet ,county4 ,these caves were formed by underground waters that moved through cracks and pores in the limestone rocks and dissolved passageways in them4 ,after the cave passages were made1 water containing dissolved calcium carbonate dripped into the cave4 ,as it evaporated1 this water left behind a deposit of calcium carbonate--the mineral calcite4 ,you can better understand how the cave deposits are formed by watching icicles grow in wet1 freezing weather4 ,first1 small hanging drops of water freeze1 and a small icicle forms4 ,then1 as more water drips over it and freezes1 the icicle grows longer and wider4 ,some of the water drips completely over the icicle and falls to the ground4 ,there1 it either freezes into a sheet of ice1 or it begins to build upward to form an upside-down icicle4 ,the water dripping down in the caves evaporates instead of freezing1 and in doing so it leaves behind a deposit of calcite4 ,illustration3 ,calcite stalactites and stalagmites in the ,caverns of ,sonora1 ,sutton ,county1 ,texas4 ,photograph courtesy of the ,travel and ,information ,division of the ,texas ,highway ,department4 ,concretions ,limestone1 shale1 and other sedimentary rocks commonly have scattered throughout them masses of other rocks and minerals1 such as limonite1 chert1 and pyrite4 ,these masses are called concretions4 ,concretions may be round or oval1 or they may have odd1 irregular shapes4 ,they--such as some of the limonite concretions of east ,texas--even may look like gourds or sweet potatoes4 ,concretions generally are harder than the surrounding rocks4 ,some are smaller than peas1 but others are several feet wide4 7,the word nodule is used to describe small1 rounded concretions as well as other small1 rounded mineral occurrences47 ,it is believed that some concretions form at the same time as the rocks in which they occur4 ,other concretions develop after the rocks themselves have formed4 ,these are deposited by underground water that contains dissolved mineral matter4 ,the water seeps through the rocks and deposits mineral matter around an object in the rock1 such as a fossil or a grain of sand1 to form a concretion4 ,geodes ,geodes are rounded1 generally hollow masses that occur mostly in limestones4 ,they are scattered through the rocks and can be lifted or dug out4 ,some geodes are as small as walnuts1 and some are as large as basketballs4 ,most of them have a rough1 dull-looking outer surface4 ,if you break geodes open1 you will find that many are lined with beautiful crystals of calcite1 celestite1 or quartz that point inward toward the hollow center4 ,illustration3 ,calcite geode found in ,lower ,cretaceous strata of western ,travis ,county1 ,texas4 ,it is thought that a geode forms when water1 carrying dissolved mineral material1 seeps into a cavity in the rock1 then deposits the mineral material as a lining in the cavity4 ,this lining becomes the outer part of the geode4 ,thus a geode--unlike a concretion1 which grows from the center outward--forms from outside to inside4 ,some of the ,lower ,cretaceous limestone rocks of ,travis1 ,williamson1 and ,lampasas counties contain calcite and celestite geodes4 ,celestite geodes have also been found in ,permian rocks in parts of ,coke1 ,fisher1 and ,nolan counties4 ,petrified ,wood ,illustration3 ,petrified wood from ,texas ,gulf ,coastal ,plain4 ,we often find some minerals occurring as petrified wood4 7,petrified wood includes silicified wood1 opalized wood1 agatized wood1 and carbonized wood47 ,petrified wood forms when plant material1 such as a tree or a bush1 is replaced by a mineral4 ,it is formed by underground water carrying dissolved mineral matter4 ,as this water seeps through sediments in which the plants are buried1 it gradually deposits agate1 chalcedony1 calcite1 opal1 chalcocite1 or some other mineral in the place of each fiber of the wood4 ,by this slow change from plant to mineral matter1 the original shape and structure of the wood remain unchanged4 ,petrified wood is commonly found in some of the ,tertiary1 ,permian1 and ,lower ,cretaceous rocks of ,texas4 7,see ,opal1 ,quartz1 ,copper ,minerals1 pp4 #gh1 #hd1 #eb74 ,,collecting ,,rocks ,,and ,,minerals ,perhaps you would like to start your own collection of rocks and minerals4 ,for this purpose you will need a hammer 7a prospector's hammer with a pick on one end of it is a good tool71 some newspapers to wrap around the specimens to keep them from breaking1 and a cloth bag in which to carry the specimens4 ,illustration3 ,prospector's hammer4 ,before you start to collect1 be sure to ask the owner's permission to go on his property4 ,if he agrees to let you come on his land1 be careful about closing gates1 and do not leave holes into which his livestock might step and be injured4 ,look out for snakes4 ,plenty of rattlers1 copperheads1 and moccasins are still left in ,texas4 ,and1 incidentally1 collecting is not allowed in ,state or ,national parks4 ,to identify the rocks and minerals that you collect1 you probably will need several articles with which to make simple tests4 ,the following can be easily obtained3 #a4 ,a pocket knife1 a copper penny1 a piece of window glass1 a steel file1 and a piece of quartz to test the hardness4 ,if you prefer to use a group of minerals of known hardness1 such as those of ,mohs scale described on pages #af-ag1 you can either collect your own or buy a prepared set from a mineral supply house4 #b4 ,a streak plate to test the color of the mineral's streak4 ,mineral streak plates can be purchased1 or a piece of unglazed tile can be used4 #c4 ,a magnifying glass to examine small cleavage surfaces1 crystals1 and rock grains4 ,a number of different kinds can be bought1 from the simple reading glass to the precisely made hand lens4 ,a lens with ten-power magnification is good for general use4 #d4 ,a small magnet to test whether or not a mineral is magnetic4 #e4 ,dilute 7#aj7 hydrochloric acid 7,h,cl71 also known as muriatic acid1 to test carbonate rocks and minerals4 ,you can buy a small bottle at a drug store4 ,be extremely careful in handling this acid1 and keep it away from small children--it is a ,,poison4 ,if you spill any on yourself1 it will burn your skin and eat holes in your clothes4 ,illustration3 ,hand lens4 ,the rock and mineral identification charts on pages #bd-da will help you to make the simple identification tests in a methodical way4 ,it is a good idea to have some system of labeling your rock and mineral specimens4 ,some collectors carry note paper with them on field trips4 ,then they can write down the location and1 if possible1 the name of the rock or mineral4 ,this information is either wrapped with the specimen or stuck to it with tape4 ,one way to label large collections is to put a small spot of paint or fingernail polish on each of the rock and mineral specimens4 ,when the paint has dried1 a number can be written on it in black ,india ink4 ,then1 on a file card1 the name and the number of the specimen can be written1 together with the place where it was found1 the date of collection1 and the name of the collector4 ,,rock ,,and ,,mineral ,,identification ,,charts ,to help you identify them1 various ,texas rocks and minerals are listed together in the following charts according to properties that they have in common4 ,although useful1 the identification charts may not always give you perfect results4 ,for example1 hardness1 which is used as a guide1 is not to be completely relied upon in the identification of rocks4 ,the charts on the following pages pertain only to the rocks and minerals that are described in this book4 ,it is quite possible that you will find rocks and minerals in ,texas that are not included in these charts4 ,if you find a rock or a mineral that you are unable to identify1 you can check your local library for reference books that may aid you 7several such references are noted on pages #ajj-aja74 ,if you need further help1 possibly the science teacher at a nearby public school will be able to identify the specimen for you4 ,or if a college or university is located in your area 7especially one that has a department of geology71 you can obtain help there4 ,in ,texas1 the ,bureau of ,economic ,geology is a mineral information center4 ,most other states have similar geological research and public-service organizations4 ,other sources of information might be the gem and mineral societies that are found in a number of communities4 ,many of the members of these organizations are experts in the identification of rocks and minerals4 ,how ,to ,use the ,mineral ,identification ,charts ,in the mineral identification charts 7pp4 #bf-ch71 the minerals have been grouped1 first of all1 on the basis of luster3 the first group includes the minerals that appear metallic and almost metallic 7submetallic72 the second group includes those that appear nonmetallic4 ,next1 the minerals have been arranged within the two groups according to color4 ,after you have determined the luster and the color of an unknown mineral1 turn to the ,key to ,mineral ,identification ,charts on page #be4 ,it will direct you to the proper mineral chart4 ,mineral ,charts #a through #e1 which include the minerals of various colors with metallic and submetallic lusters1 are subdivided according to the hardness of the minerals4 ,to determine the hardness of a mineral that has one of these lusters1 you can make the following tests3 #a4 ,will the mineral readily leave a mark on paper8 #b4 ,if it will not readily leave a mark on paper1 will an ordinary pocket knife scratch it8 #c4 ,is it too hard to be scratched by an ordinary pocket knife8 ,mineral ,charts #f through #ae are for the nonmetallic minerals of various colors4 ,they1 too1 are subdivided according to the hardness of the minerals1 as follows3 #a4 ,can the mineral be scratched by a fingernail8 #b4 ,if it cannot be scratched by a fingernail1 can it be scratched by a copper penny8 #c4 ,if it cannot be scratched by a copper penny1 can it be scratched by an ordinary pocket knife8 #d4 ,if it cannot be scratched by an ordinary pocket knife1 can it be scratched by a piece of quartz8 #e4 ,is it too hard to be scratched by quartz8 ,when the luster1 color1 and hardness of a mineral have been determined1 you may find that several minerals on the charts fit the description4 ,to narrow your choice1 you can then test other properties of the mineral4 ,notice the 8remarks0 column on the charts4 ,in it1 is mentioned anything that is distinctive about the mineral4 ,for more complete mineral identification lists and tables1 you can use textbooks1 such as ,dana's ,manual of ,mineralogy1 revised by ,c4 ,s4 ,hurlbut1 ,jr41 or ,mineralogy1 by ,e4 ,h4 ,kraus1 ,w4 ,f4 ,hunt1 and ,l4 ,s4 ,ramsdell4 ,key to ,mineral ,identification ,charts ,if the mineral has a metallic or submetallic luster1 and is3 ,consult ,mineral ,chart white #a gray #b yellow #c brown #d black #e ,if the mineral has a nonmetallic luster1 and is3 ,consult ,mineral ,chart white #f gray #g yellow #h brown #i black #aj green #aa blue #ab red or pink #ac purple or #ad violet colorless #ae ,mineral ,identification ,charts ,chart ,mineral ,streak ,remarks ,hardness ,no4 #a4 ,,metallic luster1 ,,white color ,a4 ,does not readily leave mark on paper but can be scratched by ordinary pocket knife ,native silver ,shiny silver ,silver-white #b-c white1 unless color that tarnished tarnishes to gray1 black1 or yellowish brown2 heavy2 can be flattened when hit with hammer #b4 ,,metallic or ,,submetallic luster1 ,,gray color ,a4 ,will leave mark on paper ,argentite ,shiny1 blackish ,lead-gray color #b-b to lead gray that tarnishes to dull black2 knife cuts it smoothly2 heavy2 may occur as masses and coatings ,galena ,grayish black ,shiny lead-gray #b color2 heavy2 cube-shaped fragments and crystals ,graphite ,black ,steel-gray color2 #a-b greasy feel2 very soft2 splits into thin flakes ,b4 ,does not readily leave mark on paper but can be scratched by ordinary pocket knife ,chalcocite ,grayish black ,shiny lead-gray #b-c color that tarnishes to dull black2 knife cuts it smoothly2 may have black sooty coating2 commonly occurs as compact or granular masses ,hollandite ,black ,silvery-gray #d-f color2 may occur as rounded masses ,c4 ,cannot be scratched by ordinary pocket knife ,braunite ,steel gray or ,dark steel-gray #f-f black color and submetallic luster ,hematite ,dark reddish brown ,steel-gray color2 #e-f commonly occurs 7may be as granular or softer7 compact masses2 shiny1 scaly variety is specular hematite2 notice streak ,hollandite ,black ,silvery-gray #d-f color2 may occur as rounded masses #c4 ,,metallic luster1 ,,yellow color ,a4 ,does not readily leave mark on paper but can be scratched by ordinary pocket knife ,chalcopyrite ,greenish black ,brass-yellow or #c-d golden-yellow color that may tarnish and show rainbow-like colors2 commonly massive2 notice streak ,gold ,shiny golden ,shiny yellow #b-c yellow color2 extremely heavy2 flattens when hit with hammer2 notice streak ,b4 ,cannot be scratched by ordinary pocket knife ,pyrite ,black1 greenish ,shiny1 pale #f-f black1 or golden-yellow or brownish black brass-yellow color that may tarnish2 occurs as grains1 as masses1 or as cubes or other crystal shapes2 notice hardness and streak #d4 ,,metallic or ,,submetallic luster1 ,,brown color ,a4 ,does not readily leave mark on paper but can be scratched by ordinary pocket knife ,limonite ,rusty yellowish ,dark-brown color2 #e-e brown some specimens have a shiny black surface2 notice streak ,b4 ,cannot be scratched by an ordinary pocket knife ,cassiterite ,pale brown1 pale ,brown2 #f-g yellow or white submetallic2 heavy2 notice streak ,hematite ,dark reddish brown ,dark brown color2 #e-f commonly occurs 7may be as granular or softer7 compact masses2 notice streak ,limonite ,rusty1 yellowish ,dark brown color2 #e-e brown some specimens have a shiny black surface2 notice streak #e4 ,,metallic or ,,submetallic luster1 ,,black color ,a4 ,will leave mark on paper ,argentite ,shiny1 blackish ,lead-gray color #b-b to lead grey that tarnishes to dull black2 knife cuts it smoothly2 heavy2 may occur as masses and coatings ,graphite ,black ,greasy feel2 very #a-b soft2 splits into thin flakes ,pyrolusite ,black ,very soft2 will #a-b soil fingers2 may be powdery ,b4 ,does not readily leave mark on paper but can be scratched by an ordinary pocket knife ,chalcocite ,grayish black ,shiny lead-gray #b-c color that tarnishes to dull black2 knife cuts it smoothly2 may have a black sooty coating2 commonly occurs as compact or granular masses ,hollandite ,black ,may occur as #d-f rounded masses ,limonite ,rusty1 yellowish ,some specimens #e-e brown have shiny black surface2 notice streak ,c4 ,cannot be scratched by an ordinary pocket knife ,braunite ,steel gray or ,luster is #f-f black submetallic ,cassiterite ,pale brown1 pale ,submetallic #f-g yellow1 or white luster2 heavy2 notice streak ,hematite ,dark reddish brown ,notice streak2 #e-f commonly occurs 7may be as granular or softer7 compact masses ,hollandite ,black ,may occur as #d-f rounded masses ,limonite ,rusty yellowish ,some specimens #e-e brown have shiny black surface2 notice streak ,magnetite ,black ,fragments cling #f to a magnet ,pitchblende ,brownish black ,brownish black1 #e greenish black1 or black2 radioactive2 heavy2 may appear dull or greasy #f4 ,,nonmetallic luster1 ,,white color ,a4 ,can be scratched by a fingernail ,cerargyrite ,shiny white or ,appears waxy2 #a-a gray knife cuts it smoothly2 turns violet brown to black when exposed to light ,gypsum ,white ,soft2 occurs as #b crystals or as fibrous1 granular1 compact1 or earthy masses ,talc ,white ,knife cuts it #a smoothly2 feels soapy or greasy2 splits into thin flakes ,b4 ,cannot be scratched by a fingernail but can be scratched by a copper penny ,anhydrite ,white ,commonly occurs #c-c as sugary-looking masses ,barite ,white ,rather heavy2 #c-c cleavage fragments may be flat and slab-like ,calcite ,white ,dilute #c hydrochloric acid fizzes on calcite2 perfect cleavage in #c directions gives rhomb-shaped fragments ,celestite ,white ,not quite as #c-c heavy as barite2 crystals commonly prism-shaped or flat-looking2 some cleavage fragments are flat and slab-like ,halite ,white ,salty taste2 #b dissolves in water2 cube-shaped cleavage fragments ,c4 ,cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife ,anhydrite ,white ,commonly occurs #c-c as sugary-looking masses ,barite ,white ,rather heavy2 #c-c cleavage fragments may be flat and slab-like ,celestite ,white ,not quite as #c-c heavy as barite2 crystals commonly prism-shaped or flat-looking2 some cleavage fragments are flat and slab-like ,dolomite ,white ,commonly occurs #c-d as granular masses and as rhomb-shaped crystals2 dilute hydrochloric acid may fizz slightly on dolomite ,fluorite ,white ,cleavage in #d #d directions can give fragments that are shaped like octahedrons2 crystals commonly cubes ,opal ,white ,curved1 #e-f conchoidal fracture2 may appear glassy1 greasy1 resinous1 or dull2 milky white and bluish-white precious opal shows plays of colors ,d4 ,cannot be scratched by an ordinary pocket knife but can be scratched by quartz ,feldspar ,white ,glassy or pearly #f luster2 good cleavage in #b directions that meet at an angle of #ij or near #ij2 common in granite and pegmatite rocks ,opal ,white ,curved1 #e-f conchoidal fracture2 may appear glassy1 greasy1 resinous1 or dull2 milky white and bluish-white precious opal shows plays of colors ,quartz ,white ,curved conchoidal #g fracture2 occurs as milky quartz1 chert1 and chalcedony2 crystals commonly #f-sided prisms with pyramid-like ends #g4 ,,nonmetallic luster1 ,,gray color ,a4 ,can be scratched by a fingernail ,amphibole asbestos ,white ,made up of #a-b slender1 flexible fibers that can be pulled apart ,cerargyrite ,shiny white or ,appears waxy2 #a-a gray knife cuts it smoothly2 turns violet brown to black when exposed to light ,gypsum ,white ,soft2 occurs as #b crystals or as fibrous1 granular1 compact1 or earthy masses ,sulfur ,white or pale ,will burn with a #a-b yellow blue flame2 commonly found as crystals1 crusts1 or grains ,talc ,white ,knife cuts it #a smoothly2 feels soapy or greasy2 splits into thin flakes ,b4 ,cannot be scratched by a fingernail but can be scratched by a copper penny ,amphibole asbestos ,white ,made up of #a-b slender1 flexible fibers that can be pulled apart ,anhydrite ,white ,commonly occurs #c-c as sugary-looking masses ,calcite ,white ,dilute #c hydrochloric acid fizzes on calcite2 perfect cleavage in #c directions gives rhomb-shaped fragments ,celestite ,white ,crystals commonly #c-c prism-shaped or flat-looking2 some cleavage fragments are flat and slab-like ,halite ,white ,salty taste2 #b dissolves in water2 cube-shaped cleavage fragments ,sulfur ,white or pale ,will burn with a #a-b yellow blue flame2 commonly found as crystals1 crusts1 or grains ,c4 ,cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife ,anhydrite ,white ,commonly occurs #c-c as sugary-looking masses ,celestite ,white ,crystals commonly #c-c prism-shaped or flat-looking2 some cleavage fragments are flat and slab-like ,dolomite ,white ,commonly occurs #c-d as granular masses and as rhomb-shaped crystals2 dilute hydrochloric acid may fizz slightly on dolomite ,opal ,white ,curved1 #e-f conchoidal fracture2 may appear glassy1 greasy1 resinous1 or dull ,d4 ,cannot be scratched by an ordinary pocket knife but can be scratched by quartz ,feldspar ,white ,glassy or pearly #f luster2 good cleavage in #b directions that meet at an angle of #ij or near #ij2 common in granite and pegmatite rocks ,opal ,white ,curved1 #e-f conchoidal fracture2 may appear glassy1 greasy1 resinous1 or dull ,quartz ,white ,curved conchoidal #g fracture2 occurs as chert and chalcedony #h4 ,,nonmetallic luster1 ,,yellow color ,a4 ,can be scratched by a fingernail ,carnotite ,yellow ,bright canary #b yellow or lemon yellow2 radioactive2 occurs as crusts and powdery masses ,gypsum ,white ,yellowish2 soft2 #b occurs as crystals or as fibrous1 granular1 compact or earthy masses ,limonite ,rusty yellowish ,brownish-yellow #a brown color2 may be soft and earthy ,muscovite 7white ,white ,light colored2 #b-b mica7 splits into thin1 flat1 transparent sheets that will bend without breaking ,sulfur ,white or pale ,will burn with a #a-b yellow blue flame2 commonly found as crystals1 crusts1 or grains ,uranophane ,light yellow to ,yellow to #b-c light yellow yellow-orange orange color2 radioactive ,b4 ,cannot be scratched by a fingernail but can be scratched by a copper penny ,barite ,white ,rather heavy2 #c-c cleavage fragments may be flat and slab-like ,calcite ,white ,yellowish2 dilute #c hydrochloric acid fizzes on calcite2 perfect cleavage in #c directions gives rhomb-shaped fragments ,muscovite 7white ,white ,light colored2 #b-b mica7 splits into thin1 flat1 transparent sheets that will bend without breaking ,sulfur ,white or pale ,will burn with a #a-b yellow blue flame2 commonly found as crystals1 crusts1 or grains ,uranophane ,light yellow to ,yellow to #b-c light yellow yellow-orange orange color2 radioactive ,c4 ,cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife ,barite ,white ,rather heavy2 #c-c cleavage fragments may be flat and slab-like ,opal ,white ,curved1 #e-f conchoidal fracture2 may appear glassy1 greasy1 resinous1 or dull ,d4 ,cannot be scratched by an ordinary pocket knife but can be scratched by quartz ,feldspar ,white ,glassy or pearly #f luster2 good cleavage in #b directions that meet at an angle of #ij or near #ij ,garnet ,white ,commonly occurs #f-g as crystals ,opal ,white ,curved1 #e-f conchoidal fracture2 may appear glassy1 greasy1 resinous1 or dull ,quartz ,white ,curved conchoidal #g fracture2 brownish-yellow smoky quartz crystals commonly #f-sided prisms with pyramid-like ends2 chalcedony and jasper may be a shade of yellow1 too #i4 ,,nonmetallic luster1 ,,brown color ,a4 ,can be scratched by a fingernail ,gypsum ,white ,brownish2 soft2 #b occurs as crystals or as fibrous1 granular1 compact or earthy masses ,limonite ,rusty yellowish ,may be soft and #a brown earthy ,muscovite 7white ,white ,light colored2 #b-b mica7 splits into thin1 flat1 transparent sheets that will bend without breaking ,sulfur ,white or pale ,will burn with a #a-b yellow blue flame2 commonly found as crystals1 crusts1 or grains ,b4 ,cannot be scratched by a fingernail but can be scratched by a copper penny ,barite ,white ,rather heavy2 #c-c cleavage fragments may be flat and slab-like ,biotite 7black mica7 ,white ,dark brown2 #b-c splits into thin1 flat sheets that will bend without breaking ,calcite ,white ,dilute #c hydrochloric acid fizzes on calcite2 perfect cleavage in #c directions gives rhomb-shaped fragments ,halite ,white ,salty taste2 #b dissolves in water2 cube-shaped cleavage fragments ,muscovite 7white ,white ,light colored2 #b-b mica7 splits into thin1 flat1 transparent sheets that will bend without breaking ,sulfur ,white or pale ,will burn with a #a-b yellow blue flame2 commonly found as crystals1 crusts1 or grains ,c4 ,cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife ,barite ,white ,rather heavy2 #c-c cleavage fragments may be flat and slab-like ,dolomite ,white ,commonly occurs #c-d as granular masses and as rhomb-shaped crystals2 dilute hydrochloric acid may fizz slightly on dolomite ,fluorite ,white ,cleavage in #d #d directions can give fragments that are shaped like octahedrons2 crystals commonly cube-shaped ,d4 ,cannot be scratched by an ordinary pocket knife but can be scratched by quartz ,cassiterite ,pale brown1 pale ,brown1 reddish #f-g yellow1 or white brown1 or yellowish brown2 heavy2 dull to brilliant luster ,feldspar ,white ,glassy or pearly #f luster2 good cleavage in #b directions that meet at an angle of #ij or near #ij2 common in granite and pegmatite rocks ,garnet ,white ,commonly occurs #f-g as crystals ,quartz ,white ,curved conchoidal #g fracture2 brown smoky quartz crystals commonly #f-sided prisms with pyramid-like ends2 chalcedony1 chert1 and jasper may be a shade brown1 too ,tourmaline ,white ,dark brown #g-g variety is dravite2 notice hardness1 striations on crystals1 and triangular cross section of some crystals ,e4 ,cannot be scratched by quartz ,tourmaline ,white ,dark brown #g-g variety is dravite2 notice hardness1 striations on crystals1 and triangular cross section of some crystals #aj4 ,,nonmetallic luster1 ,,black color ,a4 ,cannot be scratched by a fingernail but can be scratched by a copper penny ,biotite 7black mica7 ,white ,splits into thin1 #b-c flat sheets that will bend without breaking ,b4 ,cannot be scratched by an ordinary pocket knife but can be scratched by quartz ,garnet ,white ,commonly occurs #f-g as crystals ,quartz ,white ,curved conchoidal #g fracture2 brownish-black smoky quartz crystals commonly #f-sided prisms with pyramid-like ends2 chalcedony and chert may be black1 too ,tourmaline ,white ,black variety is #g-g schorl2 notice hardness1 striations on crystals1 and triangular cross section of some crystals ,c4 ,cannot be scratched by quartz ,tourmaline ,white ,black variety is #g-g schorl2 notice hardness1 striations on crystals1 and triangular cross section of some crystals #aa4 ,,nonmetallic luster1 ,,green color ,a4 ,can be scratched by a fingernail ,amphibole asbestos ,white ,made up of #a-b slender1 flexible fibers that can be pulled apart ,cerargyrite ,shiny white or ,light greenish #a-a gray color2 appears waxy2 knife cuts it smoothly2 turns violet brown to black when exposed to light ,muscovite 7white ,white ,light colored2 #b-b mica7 splits into thin1 flat1 transparent sheets that will bend without breaking ,sulfur ,white or pale ,greenish2 will #a-b yellow burn with a blue flame2 commonly found as crystals1 crusts1 or grains ,talc ,white ,light greenish #a color2 knife cuts it smoothly2 feels soapy or greasy2 splits into thin flakes ,b4 ,cannot be scratched by a fingernail but can be scratched by a copper penny ,amphibole asbestos ,white ,made up of #a-b slender1 flexible fibers that can be pulled apart ,biotite 7black mica7 ,white ,dark green2 #b-c splits into thin1 flat1 translucent sheets that will bend without breaking ,calcite ,white ,dilute #c hydrochloric acid fizzes on calcite2 perfect cleavage in #c directions gives rhomb-shaped fragments ,halite ,white ,greenish tint2 #b salty taste2 dissolves in water2 cube-shaped cleavage fragments ,muscovite 7white ,white ,light colored2 #b-b mica7 splits into thin1 flat1 transparent sheets that will bend without breaking ,serpentine ,white ,two kinds3 silky #b-d and fibrous1 waxy and platy ,sulfur ,white or pale ,greenish2 will #a-b yellow burn with a blue flame2 commonly found as crystals1 crusts1 or grains ,c4 ,cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife ,fluorite ,white ,cleavage in #d #d directions can give fragments shaped like octahedrons2 crystals commonly cubes ,malachite ,green ,bright green #c-d color2 dilute hydrochloric acid will fizz on malachite ,serpentine ,white ,two kinds3 silky #b-d and fibrous1 waxy and platy ,d4 ,cannot be scratched by an ordinary pocket knife but can be scratched by quartz ,feldspar ,white ,glassy or pearly #f luster2 good cleavage in #b directions that meet at an angle of #ij or near #ij ,garnet ,white ,commonly occurs #f-g as crystals #ab4 ,,nonmetallic luster1 ,,blue color ,a4 ,cannot be scratched by a fingernail but can be scratched by a copper penny ,anhydrite ,white ,commonly occurs #c-c as sugary-looking masses ,barite ,white ,rather heavy2 #c-c cleavage fragments may be flat and slab-like ,calcite ,white ,dilute #c hydrochloric acid fizzes on calcite2 perfect cleavage in #c directions gives rhomb-shaped fragments ,celestite ,white ,not quite as #c-c heavy as barite2 crystals commonly prism-shaped or flat-looking2 some cleavage fragments are flat and slab-like ,halite ,white ,salty taste2 #b dissolves in water2 cube-shaped cleavage fragments ,b4 ,cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife ,anhydrite ,white ,commonly occurs #c-c as sugary-looking masses ,azurite ,blue ,bright1 intense #c-d blue color2 dilute hydrochloric acid will fizz on azurite ,barite ,white ,rather heavy2 #c-c cleavage fragments may be flat and slab-like ,celestite ,white ,not quite as #c-c heavy as barite2 crystals commonly prism-shaped or flat-looking2 some cleavage fragments are flat and slab-like ,fluorite ,white ,cleavage in #d #d directions can give fragments that are shaped like octahedrons2 crystals commonly cube-shaped ,opal ,white ,curved1 #e-f conchoidal fracture2 may appear glassy1 greasy1 resinous1 or dull2 milky white and bluish-white precious opal shows plays of colors ,c4 ,cannot be scratched by an ordinary pocket knife but can be scratched by quartz ,feldspar ,white ,glassy or pearly #f luster2 good cleavage in #b directions that meet at an angle of #ij or near #ij ,opal ,white ,curved1 #e-f conchoidal fracture2 may appear glassy1 greasy1 resinous1 or dull2 milky white and bluish-white precious opal shows plays of colors ,quartz ,white ,curved conchoidal #g fracture2 occurs as crystalline quartz and as bluish chalcedony ,d4 ,cannot be scratched by quartz ,topaz ,white ,perfect basal #h cleavage gives flat1 plate-like fragments2 notice hardness #ac4 ,,nonmetallic luster1 ,,red or ,,pink color ,a4 ,can be scratched by a fingernail ,gypsum ,white ,reddish2 soft2 #b occurs as crystals or as fibrous1 granular1 compact1 or earthy masses ,hematite ,dark reddish brown ,brownish-red #a color2 soft and earthy ,sulfur ,white or pale ,reddish2 will #a-b yellow burn with blue flame2 commonly found as crystals1 crusts1 or grains ,b4 ,cannot be scratched by a fingernail but can be scratched by a copper penny ,anhydrite ,white ,pinkish tint2 #c-c commonly occurs as sugary-looking masses ,barite ,white ,pinkish tint2 #c-c rather heavy2 cleavage fragments may be flat and slab-like ,calcite ,white ,pink color2 #c dilute hydrochloric acid fizzes on calcite2 perfect cleavage in #c directions gives rhomb-shaped fragments ,cinnabar ,dark red ,dark red or #b bright yellowish-red color2 shiny1 brilliant luster when pure2 dull and earthy when impure2 heavy ,halite ,white ,reddish tint2 #b salty taste2 dissolves in water2 cube-shaped cleavage fragments ,sulfur ,white or pale ,reddish2 will #a-b yellow burn with blue flame2 commonly found as crystals1 crusts1 or grains ,c4 ,cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife ,anhydrite ,white ,pinkish tint2 #c-c commonly occurs as sugary-looking masses ,barite ,white ,pinkish tint2 #c-c rather heavy2 cleavage fragments may look flat and slab-like ,dolomite ,white ,pink color2 #c-d commonly occurs as granular masses and as rhomb-shaped crystals2 dilute hydrochloric acid may fizz slightly on dolomite ,fluorite ,white ,pink color2 #d cleavage in #d directions can give fragments that are shaped like octahedrons2 crystals commonly cubes ,opal ,white ,reddish color2 #e-f curved1 conchoidal fracture2 may appear glassy1 greasy1 resinous1 or dull ,d4 ,cannot be scratched by an ordinary pocket knife but can be scratched by quartz ,feldspar ,white ,glassy or pearly #f luster2 good cleavage in #b directions that meet at an angle of #ij or near #ij ,garnet ,white ,commonly occurs #f-g as crystals ,opal ,white ,reddish color2 #e-f curved1 conchoidal fracture2 may appear glassy1 greasy1 resinous1 or dull ,quartz ,white ,curved1 #g conchoidal fracture2 occurs as rose quartz1 as pink chert1 and as agate and jasper #ad4 ,,nonmetallic luster1 ,,purple or ,,violet color ,a4 ,cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife ,fluorite ,white ,cleavage in #d #d directions can give fragments that are shaped like octahedrons2 crystals commonly cubes ,b4 ,cannot be scratched by an ordinary pocket knife but can be scratched by quartz ,quartz1 variety3 ,white ,curved1 #g amethyst conchoidal fracture2 amethyst crystals commonly #f-sided prisms with pyramid-like ends #ae4 ,,nonmetallic luster1 ,,colorless ,a4 ,can be scratched by a fingernail ,cerargyrite ,shiny white or ,appears waxy2 #a-a gray knife cuts it smoothly2 turns violet brown to black when exposed to light ,gypsum ,white ,transparent #b selenite variety commonly occurs as flat1 diamond-shaped crystals2 splits into thin1 flat sheets that will not bend without breaking ,muscovite 7white ,white ,splits into thin1 #b-b mica7 flat1 transparent sheets that will bend without breaking ,b4 ,cannot be scratched by a fingernail but can be scratched by a copper penny ,barite ,white ,rather heavy2 #c-c cleavage fragments may be flat and slab-like ,calcite ,white ,dilute #c hydrochloric acid fizzes on calcite2 perfect cleavage in #c directions gives rhomb-shaped fragments ,celestite ,white ,not quite as #c-c heavy as barite2 crystals commonly prism-shaped or flat-looking2 some cleavage fragments are flat and slab-like ,halite ,white ,salty taste2 #b dissolves in water2 cube-shaped cleavage fragments ,muscovite 7white ,white ,splits into thin1 #b-b mica7 flat1 transparent sheets that will bend without breaking ,c4 ,cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife ,barite ,white ,rather heavy2 #c-c cleavage fragments may be flat and slab-like ,celestite ,white ,not quite as #c-c heavy as barite2 crystals commonly prism-shaped or flat-looking2 some cleavage fragments are flat and slab-like ,dolomite ,white ,commonly occurs #c-d as granular masses and as rhomb-shaped crystals2 dilute hydrochloric acid may fizz slightly on dolomite ,fluorite ,white ,cleavage in #d #d directions can give fragments that are shaped like octahedrons2 crystals commonly cubes ,opal ,white ,curved1 #e-f conchoidal fracture2 transparent hyalite variety resembles ice ,d4 ,cannot be scratched by an ordinary pocket knife but can be scratched by quartz ,opal ,white ,curved1 #e-f conchoidal fracture2 transparent hyalite variety resembles ice ,quartz ,white ,curved1 #g conchoidal fracture2 rock crystal quartz commonly #f-sided prism with pyramid-like ends ,e4 ,cannot be scratched by quartz ,topaz ,white ,perfect basal #h cleavage gives flat1 plate-like fragments2 notice hardness ,how ,to ,use the ,rock ,identification ,charts ,in the rock identification charts on pages #dj-da1 the ,texas rocks described in this book are arranged in four major groups according to their texture4 #a4 ,glassy 7the rocks are smooth1 dark1 and shiny7 #b4 ,compact1 dull1 or stony 7the rocks are smooth and dull1 but the individual grains are too small to be recognized7 #c4 ,granular 7at least some of the individual grains of the rocks are large enough to be seen without a magnifying glass7 #d4 ,fragmental 7the rocks are made up of fragments that are either loose or cemented together7 ,consult ,rock ,chart #a1 if the rock is glassy2 ,chart #b1 if it is compact1 dull1 or stony2 ,chart #c1 if it is granular2 and ,chart #d1 if it is fragmental4 ,two of the rock charts are subdivided4 ,in ,rock ,chart #b1 the compact1 dull1 or stony rocks are arranged according to hardness as follows3 ,a4 ,rocks that can be scratched by a fingernail ,b4 ,rocks that cannot be scratched by a fingernail but can be scratched by an ordinary pocket knife ,c4 ,rocks that cannot be scratched by an ordinary pocket knife ,in ,rock ,chart #c1 the granular rocks also are arranged according to hardness into3 ,a4 ,rocks that can be scratched by an ordinary pocket knife ,b4 ,rocks that cannot be scratched by an ordinary pocket knife ,these harder rocks are subdivided into three groups3 #a4 ,those that have grains of about equal size #b4 ,those with large easily seen grains that are scattered through a mass of finer grains #c4 ,those rocks whose grains are arranged in layers ,in the 8remarks0 column of the rock identification charts are included further tests that will aid you in identifying the rock4 ,for a more complete rock determination chart1 you can consult a textbook1 such as ,rocks and ,rock ,minerals1 by ,l4 ,v4 ,pirsson and ,a4 ,knopf4 ,rock ,identification ,charts ,chart ,rock ,remarks ,no4 #a4 ,,glassy appearance 7rock is dark1 smooth1 and shiny7 ,obsidian ,entire rock is glassy ,vitrophyre ,crystalline grains are scattered through the dark glassy mass #b4 ,,compact1 ,,dull1 ,,or ,,stony appearance 7individual grains too small to be recognized7 ,a4 ,can be scratched by a fingernail ,chalk ,dilute hydrochloric acid fizzes on it ,clay ,earthy odor when breathed on ,diatomite ,crumbly ,rock gypsum ,made up of the mineral gypsum ,soapstone ,soapy or greasy feel ,b4 ,cannot be scratched by a fingernail but can be scratched by ordinary pocket knife ,dolomite ,dilute hydrochloric acid may fizz slightly on it2 will fizz if rock is powdered ,limestone ,dilute hydrochloric acid fizzes on it ,serpentine rock ,commonly some shade of green ,shale ,breaks in flat1 thin flakes2 earthy odor ,c4 ,cannot be scratched by an ordinary pocket knife ,basalt ,dark colored and heavy ,chert ,hard1 smooth1 and porcelain-like ,rhyolite ,light to dark colored2 may show streaks or flow structure #c4 ,,granular appearance 7at least some of the individual grains are large enough to be seen without a magnifying glass7 ,a4 ,can be scratched by an ordinary pocket knife ,limestone ,dilute hydrochloric acid will fizz on it ,marble ,dilute hydrochloric acid will fizz on calcite marble1 and it may fizz slightly on dolomite marble ,rock gypsum ,made up of the mineral gypsum ,rock salt ,has a salty taste2 made up of the mineral halite ,b4 ,generally cannot be scratched by an ordinary pocket knife 7some schist is softer7 ,grains are of about equal size 7equigranular7 #a4 ,granite ,quartz and feldspar grains interlock ,pegmatite ,large interlocking grains2 commonly feldspar1 quartz1 mica ,quartzite ,rock breaks across the quartz grains ,sandstone ,rock breaks through the cement but around the sand grains ,easily seen grains are scattered through a mass of finer grains #b4 ,basalt ,dark colored1 heavy rock ,llanite ,rusty pink feldspar and blue quartz grains embedded in a brownish rock mass ,rhyolite porphyry ,light to dark colored rock2 may show streaks or flow structure ,grains are arranged in layers #c4 ,gneiss ,interlocking grains are in straight or wavy bands ,schist ,splits in thin layers2 some schist is soft enough to be scratched by a knife #d4 ,,fragmental appearance 7rocks are made up of fragments that are either loose or cemented together7 ,breccia ,angular1 gravel-size fragments that are cemented together ,conglomerate ,rounded1 gravel-size fragments that are cemented together ,coquina ,shells and shell fragments that are cemented together ,gravel ,loose fragments ,pulverulent ,loose1 powdery fragments2 dilute limestone hydrochloric acid fizzes on them ,sand ,loose fragments no larger than a pinhead ,sandstone ,sand fragments that are cemented together ,volcanic ash ,loose1 fine1 gritty particles ,illustration3 ,physiographic outline map of ,texas4 ,,descriptions ,,of ,,some ,,texas ,,rocks ,,and ,,minerals ,the pages that follow contain descriptions of ,texas rocks and minerals4 ,the descriptions are given in alphabetical order1 except that related varieties are described together4 ,for example1 agate1 amethyst1 chert1 flint1 jasper1 onyx1 and chalcedony are discussed under quartz1 because they are varieties of quartz4 ,the descriptions include the properties of the rock or mineral that will help you identify it and also include information on where the rock or mineral can be found in ,texas1 some of its uses1 and how it may have formed4 ,the chart on page #ii lists chemical composition1 specific gravity1 and hardness of various ,texas minerals4 ,agate4 ,see ,quartz4 ,agatized ,wood4 ,see ,quartz4 ,alabaster4 ,see ,gypsum4 ,albite4 ,see ,feldspar4 ,almandite4 ,see ,garnet4 ,amethyst4 ,see ,quartz4 ,amphibole ,asbestos4 ,see ,asbestos4 ,anhydrite ,anhydrite1 calcium sulfate1 is a rather soft mineral that you can scratch with a pocket knife1 although not with a fingernail4 ,it has a glassy or a pearly luster and is transparent or translucent4 ,most anhydrite is white1 but impurities cause it to be grayish1 bluish1 or reddish4 ,when rubbed across a streak plate1 anhydrite gives a white streak4 ,this mineral has an uneven fracture1 and it cleaves in three directions that are at right angles to each other4 ,it commonly occurs as rectangular cleavage fragments or as sugary crystalline masses4 ,anhydrite resembles dolomite1 limestone1 or gypsum4 ,you can use a hardness test to distinguish it from gypsum 7anhydrite is harder7 and an acid test to distinguish it from limestone and dolomite4 ,a drop of dilute hydrochloric acid will fizz when you put it on limestone or powdered dolomite4 ,on anhydrite1 the acid does not fizz4 ,illustration3 ,massive anhydrite4 ,anhydrite occurs at several places in ,texas4 ,it is1 for example1 seen in bluffs along the ,double ,mountain ,fork and the ,salt ,fork of the ,brazos ,river in north-central ,texas4 ,most of the ,texas anhydrite1 however1 occurs underground4 ,in the ,gulf ,coastal ,plain1 the anhydrite is found below the surface in salt domes4 7,salt domes are described with halite on p4 #ff and with sulfur on p4 #ia47 ,another anhydrite locality is in the subsurface ,permian basin of west ,texas4 ,oil wells drilled in this basin penetrate great1 thick deposits of massive anhydrite4 ,the anhydrite was deposited during the ,permian ,period from a sea that covered this area4 ,as the sea gradually evaporated1 the mineral matter that was dissolved in it came out of solution to form anhydrite1 halite1 and several other minerals4 ,antigorite4 ,see ,serpentine4 ,argentite4 ,see ,silver ,minerals4 ,asbestos ,asbestos is not really any one particular mineral4 ,it is the name given to several minerals that occur in masses of slender1 delicate fibers4 ,in the more typical kinds of asbestos1 these fibers--when pulled apart--resemble soft1 fluffy1 silk strings4 ,several small deposits of amphibole asbestos have been found in the ,llano uplift area of central ,texas4 ,this asbestos is a variety of the mineral tremolite1 a calcium-magnesium silicate4 ,it has fibers that break rather easily1 and it has a silky luster4 ,it is a shade of green or gray and gives a white streak when rubbed across a streak plate4 ,when you pull its fibers apart1 you actually are breaking the mineral along its two directions of perfect cleavage4 ,this amphibole asbestos is softer than other varieties of the mineral tremolite--a copper penny scratches it easily4 ,illustration3 ,greenish1 silky amphibole asbestos from northeastern ,gillespie ,county1 ,texas4 ,the asbestos occurs in veins in ,precambrian metamorphic rocks in southern ,llano ,county1 northwestern ,blanco ,county1 and northeastern ,gillespie ,county4 ,these deposits are small4 ,a variety of the mineral serpentine called chrysotile asbestos is the kind most used by industry4 ,its fibers are commonly flexible enough and strong enough to be woven into cloth4 ,this cloth is made into articles1 such as fireproof suits1 gloves1 and theater curtains4 ,some chrysotile has been found in ,precambrian metamorphic rocks in northwestern ,blanco ,county1 but it does not break into fibers fine enough or flexible enough to be called asbestos4 ,azurite4 ,see ,copper ,minerals4 ,barite ,barite1 barium sulfate1 is a fairly common mineral in ,texas4 ,it has a glassy or a pearly luster1 and it is transparent to translucent4 ,barite is colorless1 white1 brownish1 bluish1 yellowish1 or reddish4 ,when rubbed across a streak plate1 it gives a white streak4 ,it is not extremely hard--you can scratch it with a pocket knife1 although not with a fingernail4 ,barite is distinctive because of its weight and cleavage4 ,it cleaves in three directions1 and some cleavage fragments are flat or platy4 ,for a mineral with a nonmetallic luster1 barite is heavy--it has a specific gravity of #d4e4 ,illustration3 ,barite cleavage fragment from west ,texas4 ,barite commonly occurs as prism-shaped and as flat crystals1 as granular masses1 as cleavable masses1 and as rounded masses called nodules4 ,in ,texas1 some of it was deposited in sedimentary rocks by underground waters4 ,as the waters seeped through these rocks1 mineral matter came out of solution to form the barite4 ,some of the barite in ,texas also formed from solutions that came from hot magmas4 ,a number of barite deposits have been found in ,texas1 but many of them are small4 ,barite occurs in ,precambrian metamorphic rocks in ,gillespie and ,llano counties1 in ,pennsylvanian shale in ,brewster ,county1 in ,permian shales in ,baylor and ,taylor counties1 and in ,permian limestones in ,culberson ,county4 ,it is found in ,triassic red shales in ,howard ,county and in ,cretaceous sedimentary rocks in ,brewster1 ,brown1 ,hudspeth1 ,jeff ,davis1 ,kinney1 and ,val ,verde counties4 ,in ,live ,oak ,county1 barite occurs in ,tertiary bentonitic clays4 ,barite is being mined from a deposit in the ,seven ,heart ,gap area northeast of ,van ,horn in ,culberson ,county4 ,barite is used in a number of ways4 ,it is a source of barium chemicals1 and it also is powdered and used as an ingredient in paint4 ,the oil industry uses large amounts of barite4 ,in drilling for oil by the rotary method1 water and muds are pumped down the hole to aid drilling4 ,barite is added to these drilling fluids to make them heavy1 since high-pressure gases are not as likely to blow heavy fluids out of the hole4 ,basalt ,basalt is a heavy igneous rock that is black1 dark gray1 or dark brown4 ,this rock is made up chiefly of a feldspar mineral1 such as labradorite1 and a pyroxene mineral1 such as augite4 ,other minerals may be present4 ,the mineral grains of some basalts are so small that you cannot distinguish them even with a magnifying glass4 ,other basalts1 however1 are porphyritic1 which means that they contain larger1 easily seen crystals and grains of feldspar and pyroxene scattered either through a mass of the small mineral grains or through glassy material4 ,some basalts contain many small holes4 ,these holes1 called vesicles1 were formed when bubbles of gas were trapped in the hardening magma4 ,later1 solutions moving through the rocks may have deposited another mineral--such as calcite or chalcedony--in some of the vesicles4 ,basalt forms from molten rock material that hardens either on or beneath the surface--it can be extrusive or intrusive4 ,much of the basalt now found in the ,trans-,pecos country of west ,texas formed from lava that flowed out onto the surface during the ,tertiary ,period4 ,a few of the places where basalt occurs in west ,texas are the ,chinati ,mountains of ,presidio ,county1 the ,chisos ,mountains of ,brewster ,county1 the ,davis ,mountains of ,jeff ,davis ,county1 and the ,van ,horn ,mountains of ,culberson and ,hudspeth counties4 ,illustration3 ,basalt from ,brewster ,county1 ,texas4 ,several varieties of basalt occur in the ,balcones fault region of ,bandera1 ,comal1 ,hays1 ,kinney1 ,medina1 ,travis1 and ,uvalde counties4 ,these basalts formed from molten magma that forced its way into rocks just below the earth's surface4 ,some basalt1 which is known commercially as trap rock1 is produced in ,uvalde ,county4 ,it is crushed and used for railroad ballast1 road building material1 and as concrete aggregate4 ,bentonite4 ,see ,clay4 ,biotite4 ,see ,mica4 ,braunite4 ,see ,manganese ,minerals4 ,calcite ,calcite1 calcium carbonate1 is one of the most abundant minerals in ,texas4 ,it is the chief mineral in limestone and in some marble4 ,it also serves as the cementing material in many sandstones4 ,crystals1 grains1 and cleavable masses of calcite1 which have been deposited by underground water1 occur in cracks and cavities in many of the igneous1 metamorphic1 and sedimentary rocks of ,texas4 ,calcite also occurs as cave1 spring1 and stream deposits and as caliche4 ,calcite is transparent or translucent1 and--depending on the variety--its luster is glassy to earthy4 ,most calcite is white or colorless1 but it can be a shade of pink1 blue1 green1 brown1 yellow1 or gray4 ,it gives a white streak when you rub it across a streak plate4 ,two properties of calcite to notice are the hardness and the cleavage4 ,this mineral cleaves perfectly in three directions that are not at right angles to each other1 and some of the cleavage fragments are rhombohedrons4 ,calcite is rather soft--you can scratch it with a copper penny but not with a fingernail4 ,a drop or two of dilute hydrochloric acid also will help you to identify this mineral4 ,the acid will readily fizz and bubble when it is placed on calcite4 ,illustration3 ,calcite has perfect rhombohedral cleavage4 ,the three directions of cleavage are not at right angles to each other4 ,calcite occurs in more different kinds of crystal shapes than any other mineral4 ,some of these crystals are flat and tabular2 some are rhombohedrons2 some are prisms4 ,pointed crystals1 called dog-tooth spar1 and twinned crystals have been found in the ,terlingua area of ,brewster ,county in west ,texas4 ,a somewhat unusual occurrence of calcite crystals is in geodes4 ,some of these are found in ,lower ,cretaceous rocks west of ,austin in ,travis ,county4 ,transparent crystals and cleavage fragments of calcite show a property called double refraction 7other minerals show it1 too74 ,to test this property1 you can mark a single dot on paper4 ,when you look at the dot through a piece of clear calcite1 you will see two dots instead of one4 ,this happens because a ray of light is bent 7refracted7 and is split into two rays as it enters the mineral4 ,these two rays travel through the calcite in slightly different directions1 and each carries an image of the dot through the mineral4 ,the two images that you see are at the points where the two rays leave the calcite4 ,calcite that is deposited at springs1 along river and creek banks1 and in caves is known as travertine4 ,cave forms of travertine1 including stalagmites and stalactites1 occur in several caves in ,texas4 ,another kind of travertine is called calcareous tufa or calcareous sinter4 ,it is a porous and spongy-looking material deposited from water carrying dissolved limestone and is found around the openings of some springs and along some creek and river banks4 ,a dull1 earthy calcite deposit1 known as caliche1 occurs in areas of ,texas that have scant rainfall1 such as the ,high ,plains1 west ,texas1 and the southwestern part of the ,gulf ,coastal ,plain4 ,caliche commonly is found mixed with other materials1 such as clay1 sand1 or gravel4 ,this substance may be firm and compact or loose and powdery4 ,it is thought that caliche forms when ground moisture1 containing dissolved calcium bicarbonate1 moves upward4 ,in dry areas of the country1 this moisture evaporates4 ,as it does1 it leaves a crust of calcium carbonate in the form of caliche on or near the surface of the ground4 ,illustration3 ,calcite crystals 7dog-tooth spar7 from the ,terlingua area of ,brewster ,county1 ,texas4 ,caliche is quarried in many counties in ,texas and is used chiefly as road material and as an aggregate4 ,caliche4 ,see ,calcite4 ,carnotite4 ,see ,uranium ,minerals4 ,cassiterite ,cassiterite1 tin dioxide1 is the mineral that serves as the chief source of tin4 ,tin does not corrode and tarnish1 and one of its main uses is in the making of tin cans4 7,actually1 our tin cans are made from thin sheets of steel that have been coated with a protective layer of tin47 ,cassiterite has either a nonmetallic or a submetallic luster4 ,some specimens are brilliant and shiny2 others are dull4 ,cassiterite may be translucent to transparent4 ,it may be black1 brown1 gray1 reddish brown1 or yellowish brown4 ,when rubbed across a streak plate1 this mineral leaves a pale brown1 a pale yellow1 or a white streak4 ,cassiterite is quite heavy--it has a specific gravity of #f4h to #g4a4 ,it is too hard to be scratched by an average pocket knife4 ,sometimes1 prospectors use a chemical test to help them identify cassiterite4 ,they put small pieces of metallic zinc into a jar or test tube containing dilute hydrochloric acid4 ,then they add a few fragments of the mineral that they suspect is cassiterite4 ,if the fragments are cassiterite1 they become covered with a pale gray coating of metallic tin4 ,cassiterite's most common crystal shape is a short1 #h-sided prism with pyramids at each end1 but perfect crystals are not often found4 ,most ,texas cassiterite does not show a crystal shape4 ,instead1 it occurs as crystalline masses in igneous rocks and as loose pebbles that have weathered out of these rocks4 ,cassiterite occurs in a number of places in the ,united ,states but not in large quantities4 ,a small amount of cassiterite has been found in quartz veins in ,precambrian granite in both central ,texas and west ,texas4 ,in ,el ,paso ,county1 the cassiterite is found on the east side of the ,franklin ,mountains a few miles north of ,el ,paso1 where some of it has been mined4 ,in central ,texas1 cassiterite occurs in the ,streeter area of ,mason ,county4 ,when the granite rocks in these areas were formed1 probably not all of the hot magmas cooled and hardened at the same time4 ,the fluids given off by the remaining magmas contained tin and several other elements4 ,it is believed that these fluids moved up into cracks in the granite rocks and formed the cassiterite4 ,celestite ,celestite is a strontium sulfate mineral4 ,it is colorless1 white1 yellow1 or gray4 ,light blue specimens of this mineral also are found1 and it is because of this sky-like color that celestite gets its name4 ,the word celestite comes from the ,latin word caelestis1 meaning of the sky4 ,celestite has a glassy to a pearly luster1 and it is either transparent or translucent4 ,it gives a white streak when rubbed across a streak plate4 ,celestite has a specific gravity of #c4ie to #c4ig4 ,it is1 however1 lighter than barite1 a mineral that it resembles4 ,celestite is not very hard--a knife will scratch it1 although your fingernail will not4 ,it cleaves in three directions1 and some of the fragments are flat and slabby4 ,illustration3 ,celestite cleavage fragment from ,lampasas ,county1 ,texas4 ,celestite occurs commonly either as prism-shaped or flat crystals and as cleavable1 granular1 or fibrous crystalline masses4 ,in ,texas1 it is found in geodes1 as rounded nodules1 or as bedded or layer-like deposits in limestones and other sedimentary rocks4 ,in ,real ,county1 celestite occurs on the walls of a cave in ,cretaceous limestone4 ,some celestite may be deposited by sea water1 but much of the ,texas celestite is believed to have been deposited by underground water that seeped through cracks and pores in the limestones and other sedimentary rocks4 ,this water picked up and dissolved strontium compounds that were scattered in small amounts through the rocks4 ,then1 it re-deposited the strontium in the rocks as celestite4 ,in ,texas1 beds of celestite occur in ,permian rocks in ,coke1 ,fisher1 and ,nolan counties and in ,lower ,cretaceous rocks in ,brown1 ,comanche1 and ,mills counties4 ,celestite geodes and nodules are found in ,lower ,cretaceous limestone rocks in ,lampasas1 ,travis1 and ,williamson counties1 and in ,permian rocks in ,coke1 ,fisher1 ,nolan1 and ,taylor counties4 ,celestite is one of two minerals 7the other mineral is strontianite1 strontium carbonate7 used as a source of strontium4 ,strontium compounds give a crimson-red color to a flame1 so they are used in fireworks1 tracer bullets1 and flares4 ,perhaps you have seen a red flare set out on the highway at night to warn motorists that a truck has stalled4 ,the chances are good that the flare's red flame was due to a strontium compound4 ,some of the ,texas celestite has been mined1 but most of the strontium minerals now used in the ,united ,states are imported from ,england and ,mexico4 ,cerargyrite4 ,see ,silver ,minerals4 ,chalcedony4 ,see ,quartz4 ,chalcocite4 ,see ,copper ,minerals4 ,chalcopyrite4 ,see ,copper ,minerals4 ,chalk4 ,see ,limestone4 ,chert 7,flint74 ,see ,quartz4 ,chrysotile4 ,see ,asbestos2 ,serpentine4 ,cinnabar ,cinnabar1 which is mercuric sulfide1 is the most common mercury mineral4 ,it has a dark red or a bright yellowish-red color and is transparent to translucent4 ,when rubbed across a streak plate1 it leaves a dark red streak4 ,if pure1 cinnabar has a brilliant1 shiny1 nonmetallic luster4 ,it is1 however1 commonly found mixed with impurities1 such as clay1 calcite1 iron oxide1 or bituminous material1 and then it looks dull and earthy4 ,cinnabar is quite heavy--it has a specific gravity of #h4aj4 ,it is rather soft1 and you can scratch it with a copper penny4 ,some prospectors use a quick chemical test to identify cinnabar4 ,they rub a clean1 shiny copper coin with a mineral sample that has been moistened with a drop or two of dilute hydrochloric acid4 ,if the sample is cinnabar1 a light silvery-gray coating appears on the coin4 ,cinnabar occurs as small crystals or as fine-grained or compact crystalline masses4 ,it is found in veins that fill cracks in rocks and also occurs as crusts and coatings on rocks4 ,it also may be widely scattered through rocks1 such as limestones4 ,cinnabar occurs in the ,terlingua area of ,brewster and ,presidio counties in west ,texas4 ,it has been mined there1 off and on1 since about #ahid1 and during this time1 mercury worth many millions of dollars has been produced4 ,most of this west ,texas cinnabar is found in cracks1 pores1 and breccia-filled cavities of ,cretaceous limestones and clays4 ,if you will look at the ,texas geologic map 7pp4 #d-e71 you will see that igneous rocks occur in this district4 ,many millions of years ago during the ,tertiary ,period1 when these igneous rocks were still hot magma1 some of them pushed up under the ,cretaceous rocks and emitted fluids containing mercury4 ,the fluids moved upward through cracks and pores in the ,cretaceous rocks where they deposited the mercury as cinnabar and as other mercury minerals4 ,illustration3 ,cinnabar crystals 7dark7 with calcite crystals 7white7 from the ,terlingua area of ,brewster ,county1 ,texas4 ,mercury is an unusual element4 ,instead of occurring as a solid metal at ordinary room temperatures1 as do gold1 silver1 and lead1 it remains a liquid until it is cooled to #ch degrees below zero ,fahrenheit4 ,because the silvery little drops of liquid mercury roll about as if they were alive1 this element long has been called quicksilver4 ,mercury is used in a variety of ways4 ,in some noiseless light-switches1 a glass tube containing a small ball of mercury tilts when the switch is turned 8on40 ,the mercury then rolls to the end of the tube that contains electrical contacts and quietly completes the electrical circuit4 ,in other uses1 mercury is added to silver1 tin1 and other metals to make fillings for teeth4 ,some medicines1 such as calomel and mercurochrome1 contain mercury4 ,fulminate of mercury helps to set off dynamite and other explosives4 ,mercury is used in many barometers and thermometers1 and farmers use mercury poisons to control insects and fungi4 ,mercury also commonly is used to obtain gold from its ores4 ,one method of accomplishing this is to pass wet gold-bearing gravel or crushed rock over metal plates that are coated with mercury4 ,the gold particles quickly mix with the mercury to form an amalgam1 which later can be scraped off the plates4 ,the gold is then recovered by heating the amalgam to drive off the mercury4 ,clay ,clay is a smooth1 soft1 earthy rock made up of mineral particles no bigger than specks of dust4 ,some of the particles are clay minerals1 which consist of aluminum1 silicon1 and other elements4 ,in addition1 tiny particles of quartz1 calcite1 and other minerals may also be present in the clay4 ,the clay particles are all that remain of rocks and of minerals1 such as feldspar1 that have been broken into fragments or altered into clay minerals by weathering4 ,some clay remains at the place where it formed1 but some is carried away and deposited elsewhere4 ,clay is white1 tan1 brown1 red1 green1 blue1 gray--almost any color4 ,when moist1 it has an earthy odor4 ,you can moisten a piece of clay enough to notice this just by breathing on it4 ,most clays1 when wet1 can be molded into many different shapes--that is1 they are plastic1 but when they are dry1 they are firm and solid4 ,clay is abundant in ,texas and has a number of uses4 ,some goes to make portland cement1 and some is baked or burned in a kiln to make brick1 tile1 sewer pipes1 pottery1 and other products4 ,this kind of clay is obtained from ,tertiary formations of the ,gulf ,coastal ,plain1 from ,upper ,cretaceous formations in central ,texas1 and from ,pennsylvanian formations in north-central ,texas4 7,you can locate ,tertiary1 ,cretaceous1 and ,pennsylvanian rocks on the ,texas geologic map1 pp4 #d-e47 ,a special kind of white burning clay that can be used to make chinaware is called kaolin or china clay4 ,it contains particles of the clay mineral kaolinite as well as several other clay minerals4 ,deposits of china clay occur in southern ,jeff ,davis ,county and in ,real ,county near ,leakey1 but none is being produced4 ,another kind of clay1 bentonite1 forms from weathered volcanic ash4 ,bentonite contains the clay mineral montmorillonite and looks smooth and soap-like4 ,fresh samples of this clay are white1 pale green1 or pale blue1 but dried-out or weathered samples are tan1 brown1 yellow1 or reddish4 ,when wet1 bentonite absorbs water1 swells1 and then has a jelly-like appearance4 ,surface deposits of bentonite occur chiefly in ,eocene ,tertiary formations of the ,gulf ,coastal ,plain1 in ,cretaceous formations of the ,big ,bend area of west ,texas1 and in ,quaternary formations of the ,high ,plains4 ,illustration3 ,bentonite is used as a drilling-fluid additive in the rotary method of drilling for petroleum and gas4 ,some bentonite is used to absorb unwanted coloring material in petroleum and in vegetable oils4 ,it is then known as a bleaching clay4 ,bentonite bleaching clay is obtained from some of the ,tertiary formations along the ,texas ,gulf ,coastal ,plain4 ,it has been produced in ,angelina1 ,fayette1 ,gonzales1 ,jasper1 ,walker1 and other counties in this area4 ,another important use of bentonite1 and of other clay1 too1 is as drilling mud4 ,in the rotary method of drilling for oil and gas1 mud is pumped down into the drilled hole4 ,this mud carries the rock cuttings up to the surface1 it cools the drilling tools1 and it coats and seals the walls of the hole4 ,along the ,gulf ,coastal ,plain1 drilling clay is obtained from ,tertiary formations4 ,common ,opal4 ,see ,opal4 ,copper ,minerals 7,chalcocite1 ,chalcopyrite1 ,malachite1 ,azurite7 ,a number of minerals containing copper1 such as chalcocite1 chalcopyrite1 malachite1 and azurite1 occur in small deposits in ,texas4 ,they are found chiefly in the ,llano uplift area of central ,texas1 in the ,van ,horn area of ,culberson and ,hudspeth counties in west ,texas1 and in a group of counties in north-central ,texas4 ,copper is an important element4 ,because it is an unusually good conductor of electricity 7only silver1 which costs much more1 is a better one71 it is used for many kinds of wires for switchboards1 generators1 motors1 telephone and telegraph equipment1 and light and power lines4 ,manufacturers commonly combine copper with other elements4 ,for example1 some copper is mixed with zinc to make brass and with tin and a little zinc to make bronze4 ,these mixtures are called alloys4 ,many products are made from copper alloys1 including tubing1 pipes1 jewelry1 pots1 and pans4 ,even our coins contain copper4 ,sometimes1 a prospector uses a chemical test to find out if copper is present in a mineral4 ,first1 he crushes a small sample of what he believes is a copper mineral 7such as chalcocite1 chalcopyrite1 azurite1 or malachite74 ,he then puts the sample in a glass jar or test tube and pours in a small amount of dilute nitric acid 7this acid1 like hydrochloric acid1 is poisonous74 ,after the sample has dissolved in the acid1 he adds enough ammonium hydroxide to make the solution alkaline4 ,if the sample is a copper mineral1 the solution turns a deep-blue color4 ,one of the copper minerals1 chalcocite1 copper sulfide1 also is known as copper glance4 ,it is a metallic mineral that commonly tarnishes to a dull black4 ,by chipping off a fragment to obtain a fresh surface1 you will see that it has a shiny lead-gray color4 ,chalcocite is rather soft1 and it is sectile1 that is1 a knife will cut through it as well as scratch it4 ,when you rub chalcocite across a streak plate1 it gives a grayish-black streak4 ,this mineral commonly occurs as compact masses or as granular masses4 ,chalcocite1 with its dark color1 does not look at all like copper1 which is a bright reddish brown4 ,chalcocite1 however1 is the chief copper mineral at the most important copper mine in ,texas1 the ,hazel mine1 which is about #ae miles northwest of ,van ,horn in ,culberson ,county in west ,texas4 ,this mine1 although now idle and almost filled with water1 has produced about one and a half million pounds of copper along with more valuable silver ores4 ,here1 the chalcocite and other minerals occur in material that fills large cracks in red sandstone of the ,precambrian ,hazel ,formation4 ,it is thought that long ago1 molten igneous rock material far below the surface sent out hot solutions containing copper and other elements4 ,these solutions moved upward and deposited minerals in the fracture zone in the sandstone4 ,chalcocite occurs also in north-central ,texas4 ,it is found in ,archer1 ,baylor1 ,clay1 ,foard1 ,hardeman1 ,king1 ,knox1 ,stonewall1 and several other counties of this area4 ,here1 it occurs in ,permian sedimentary rocks 7called 8red beds07 as rounded masses1 as scattered grains1 and as petrified wood4 ,because these deposits are far from any igneous rocks1 they apparently did not form in the same way as those at the ,hazel mine4 ,these north-central ,texas deposits have never really been commercially developed4 ,during the ,civil ,war1 however1 some copper from this area was made into percussion caps for the ,confederacy4 ,illustration3 ,the ,hazel copper-silver mine1 ,culberson ,county1 ,texas1 as it appeared in #aiea4 ,photograph by ,p4 ,t4 ,flawn4 ,another copper mineral1 chalcopyrite1 is a copper-iron sulfide4 ,it also is known as copper pyrites and yellow copper ore4 ,this mineral has a metallic luster and a brass-yellow or a golden-yellow color4 ,when rubbed across a streak plate1 it gives a greenish-black streak4 ,chalcopyrite will tarnish and then has bronze1 blue1 purple1 and other rainbow-like colors4 ,this mineral is fairly soft--you can scratch it with a pocket knife4 ,because of chalcopyrite's yellow color1 it has often been mistaken for gold4 ,for this reason1 it1 like iron pyrite1 is often called fool's gold4 7,see ,gold1 p4 #fj1 for ways to tell them apart47 ,chalcopyrite commonly is found in compact masses that show no crystal shapes4 ,these masses either are scattered through rocks or occur in material that fills cracks in rocks4 ,some chalcopyrite is found in ,precambrian sandstone at the ,hazel mine and in other deposits in the ,van ,horn area of ,culberson and ,hudspeth counties4 ,it also occurs in ,precambrian rocks at the ,sheridan and ,pavitte prospects in ,burnet ,county4 ,these chalcopyrite localities are in districts where igneous rocks occur4 ,it is likely that1 long ago1 hot solutions containing copper moved upward1 out of deeply buried molten magma4 ,while still far below the surface1 the solutions deposited the chalcopyrite in cracks and other openings in the nearby rocks4 ,two copper minerals of ,texas1 azurite and malachite1 are copper carbonates4 ,azurite is commonly called chessylite and blue copper2 malachite is called green copper carbonate4 ,because these minerals are carbonates1 a drop of dilute hydrochloric acid will fizz and bubble when placed on either of them4 ,azurite has a bright1 intense blue color and leaves a blue streak when rubbed across a streak plate4 ,malachite has a bright green color and leaves a green streak4 ,these minerals have a nonmetallic luster and a glassy to dull appearance4 ,commonly1 they are translucent1 although some specimens of azurite are transparent4 ,both azurite and malachite are fairly soft--a pocket knife will scratch them1 but a copper penny will not4 ,azurite and malachite occur as individual crystals1 but you are more likely to find them as crusts on rocks and on other minerals4 ,malachite is also found in rounded fibrous masses that resemble bunches of grapes 7described then as botryoidal74 ,both azurite and malachite are formed in the same way4 ,underground waters seep through rocks that contain deposits of copper minerals 7such as chalcocite and chalcopyrite7 and cause chemical reactions which change these minerals into malachite and azurite4 ,malachite is more plentiful than azurite1 but both minerals can be found together4 ,you can expect to find at least one of them at the same localities where chalcocite1 chalcopyrite1 and other copper minerals occur4 ,coquina4 ,see ,limestone4 ,diatomite4 ,see ,opal4 ,dolomite ,dolomite is the name given both to a rock and to a mineral4 ,the mineral is a calcium-magnesium carbonate and has a glassy or a pearly luster4 ,it is any of a number of colors1 such as white1 pink1 brown1 or gray1 or it can be colorless4 ,dolomite leaves a white streak on a streak plate and is transparent to translucent4 ,it is not particularly hard and can be scratched with a pocket knife1 although not with a copper penny4 ,dolomite cleaves perfectly in three directions1 and some of the cleavage fragments are rhombohedrons4 ,however1 the cleavages of the individual mineral grains in specimens of fine-grained massive dolomite are not readily distinguishable4 ,illustration3 ,dolomite rock from the vicinity of ,fairland1 ,burnet ,county1 ,texas4 ,most ,texas dolomite occurs as coarse-1 medium-1 and fine-grained crystalline masses as the chief mineral in dolomite rock and in dolomitic marble4 ,it is also found as #f-sided crystals that are rhomb-shaped2 when the faces are curved1 they have a saddle-like appearance4 ,crystals of the mineral dolomite commonly occur in cavities in the dolomite rocks4 ,it is believed that they were deposited there by seeping underground waters4 ,the waters dissolved some of the dolomite in the rocks and then re-deposited it as crystals4 ,dolomite rock is made up mostly of crystalline grains of the mineral dolomite4 ,in addition1 quartz grains1 calcite1 and other minerals may be present4 ,dolomite rock is almost any color--white1 buff1 pink brown1 gray4 ,it resembles some limestone1 and these two rocks actually are closely related4 ,to help tell them apart1 dilute hydrochloric acid often is used4 ,a few drops of this acid will readily fizz and bubble if the rock you put them on is a limestone4 ,if the rock is dolomite1 the acid will effervesce only very little or not at all4 7,if1 however1 the acid is put on powdered dolomite1 it then will fizz readily47 ,dolomite is slightly harder than limestone1 and it also is slightly heavier4 ,some dolomite rocks formed directly from materials that were dissolved in sea water1 and others are altered limestone rocks4 ,some limestones altered into dolomite on the sea floor by the addition of magnesium from the sea water4 ,others changed into dolomite much later after the sea had withdrawn and the limestones had become a part of the land2 underground waters containing magnesium seeped through these limestones and altered them into dolomite4 ,many of the dolomite rocks are found with limestones4 ,in ,texas they occur mostly in ,cambrian1 ,ordovician1 ,mississippian1 ,pennsylvanian1 ,permian1 and ,cretaceous formations4 ,the geologic map 7pp4 #d-e7 indicates where these strata appear at the surface in ,texas4 ,dolomite is abundant in the ,llano uplift area of central ,texas--particularly in the ,cambrian and ,ordovician rocks4 ,a number of these central ,texas dolomites have been quarried for use as building stones4 ,some of them also have been crushed and used as a road-building material and as a stone aggregate that is mixed with cement to make concrete4 ,this dolomite is also used as terrazzo chips 7terrazzo floors are described with serpentine on p4 #hh74 ,in addition1 ,ellenburger 7,ordovician7 dolomite from ,burnet ,county was used during ,world ,war ,,ii as a source of the lightweight metal magnesium4 ,dravite4 ,see ,tourmaline4 ,feldspar ,feldspar is the name given to a group of nonmetallic minerals that are much alike4 ,several of them are so similar that a petrographic microscope must be used to tell them apart4 ,each of the feldspar minerals is an aluminum silicate4 ,each of them contains1 in addition1 at least one of the following elements3 potassium1 sodium1 calcium1 and barium4 ,the feldspar minerals that are found in ,texas include albite1 a sodium-aluminum silicate1 and orthoclase and microcline1 which are both potassium-aluminum silicates4 ,the feldspar minerals are transparent to translucent and have either glassy or pearly lusters4 ,they can be white1 cream1 or a shade of red1 brown1 yellow1 blue1 gray1 or green4 ,when you rub a feldspar across a streak plate1 it leaves a white streak4 ,the feldspars are rather hard--a pocket knife will not scratch them1 although a piece of quartz or a steel file will4 ,these minerals have good cleavage in two directions4 ,the cleavages meet at an angle of about #ij1 so that the cleavage fragments have square corners4 ,illustration3 ,feldspar cleavage fragment from ,llano ,county1 ,texas4 ,the two directions of good cleavage meet at an angle of about #ij4 ,the feldspars are important rock-forming minerals4 ,you can find them in igneous rocks1 such as granite or pegmatite1 and in metamorphic rocks1 such as gneiss4 ,they also occur as fragments in sedimentary rocks1 such as some sandstone and conglomerate4 ,although the feldspars can originate in other ways1 they form mostly from hot magmas that cool and crystallize into igneous rocks4 ,these minerals occur in the rocks as grains1 as cleavable masses1 and as individual crystals4 ,the crystals may be shaped like prisms1 or they may be flat and slabby4 ,good places to look for feldspars are in areas where granites1 pegmatites1 and other intrusive igneous rocks appear at the surface4 ,the pegmatite rocks of ,burnet1 ,gillespie1 ,llano1 and ,mason counties in the ,llano uplift area of central ,texas1 and those of the ,van ,horn ,mountains in ,hudspeth and ,culberson counties in west ,texas1 are especially good sources of feldspar4 ,large cleavable masses and crystals that are more than a foot long are found in some of these rocks4 ,the feldspars have a number of uses4 ,some of the pegmatite feldspars from ,llano ,county in central ,texas have been crushed and used as granules for built-up and composition roofs4 ,in addition1 some have been shipped to ,mexico for glass-making4 ,some of the other uses of feldspar are in making porcelain1 ceramic glazes1 and scouring compounds4 ,a few of the feldspar minerals1 such as the variety of microcline known as amazonstone1 are used as gemstones4 ,illustration3 ,microcline feldspar crystals from near ,granite ,shoals ,lake1 ,llano ,county1 ,texas4 ,fibrous ,gypsum4 ,see ,gypsum4 ,flint4 ,see ,quartz4 ,fluorite ,fluorite is calcium fluoride4 ,the fluorite that is mined and sent to market1 however1 commonly is found mixed with quartz1 calcite1 limestone1 or other rocks and minerals4 ,industry calls this mixture fluorspar4 ,fluorite is a transparent to translucent mineral that has a glassy luster4 ,it may be colorless1 or it may be white1 pink1 green1 purple1 brown1 or blue4 ,some specimens show more than one color4 ,when you rub fluorite across a streak plate1 it leaves a white streak4 ,this mineral is not particularly hard--a pocket knife will scratch it1 although a copper penny will not4 ,fluorite has perfect cleavage in four directions4 ,by carefully breaking a specimen1 you can obtain cleavage fragments that are shaped like octahedrons4 ,fluorite occurs as cleavable masses1 as fine or coarse grains1 and as crystals4 ,most of the crystals are cubes1 but some may be octahedrons1 dodecahedrons1 or combinations of these4 ,fluorite has been found both in west ,texas and in central ,texas4 ,in the ,llano uplift area of central ,texas1 it occurs in a number of ,precambrian granite1 pegmatite1 schist1 and gneiss rocks4 ,the most important1 although small1 deposit in this area is near ,spring ,creek a few miles west of ,burnet in ,burnet ,county4 ,here1 prospectors have dug holes and pits in gneiss and schist rocks and found layers of fluorite in them4 ,the largest known fluorite deposits in ,texas 7they are not particularly large when you compare them with the deposits in ,illinois and ,kentucky7 are those in the ,eagle ,mountains of ,hudspeth ,county4 ,this fluorite occurs in both igneous and sedimentary rocks4 ,many years ago1 probably during the late part of the ,tertiary ,period1 hot magma far below the surface gave off liquids and gases containing fluorine4 ,these fluids moved up through large cracks 7called faults7 in ,cretaceous limestones and ,tertiary igneous rocks and deposited fluorite in them4 ,in places1 beds of limestone have been replaced by fluorite4 ,some of this west ,texas fluorite has been mined and shipped to market4 ,illustration3 ,fluorite has octahedral cleavage4 ,the four directions of perfect cleavage can result in cleavage fragments that are octahedrons4 ,fluorite is extremely important as a flux in steel-making to help the ingredients of the molten steel blend together4 ,in addition1 it combines with sulfur1 phosphorus1 and other unwanted substances so that they can be removed from the steel4 ,other important uses of fluorite are in glass-making and in the manufacture of hydrofluoric acid4 ,this acid is used in the aluminum industry as well as in industries that make high-octane gasoline1 insecticides1 and refrigerants for refrigerators and freezers4 ,galena ,galena1 lead sulfide1 is a shiny1 lead-gray1 metallic mineral that has a specific gravity of #g4d to #g4f4 ,it is soft enough to mark paper1 and it leaves a grayish-black streak on a streak plate4 ,this mineral cleaves perfectly in three directions1 and the cleavage fragments have square corners--some are cubes4 ,galena occurs as cleavable masses1 as fine or coarse grains1 and as crystals1 most of which are cubes4 ,galena commonly is associated with other minerals2 for example1 some of the west ,texas galena either contains some silver 7then called argentiferous galena7 or occurs with it4 ,sphalerite1 a zinc mineral1 is commonly found with galena4 ,galena is an important mineral because it is the chief source of lead4 ,compounds of lead1 called white lead1 red lead1 and litharge1 are used as paint pigments4 ,automobile batteries contain lead plates1 and tetraethyl lead is added to gasoline to keep the car's motor from knocking4 ,some other uses of lead are in bullets1 type metal1 solder1 and cable coverings4 ,illustration3 ,galena has perfect cubic cleavage4 ,the three directions of cleavage are at right angles to each other resulting in cubic cleavage fragments4 ,galena has been found in several areas of ,texas and has been mined in central and west ,texas4 ,none1 however1 has been produced in recent years4 ,most of the galena mined in west ,texas was obtained from silver mines1 where the galena was a by-product4 ,some of the west ,texas galena deposits are at ,altuda ,mountain east of ,alpine in ,brewster ,county1 in the ,eagle ,mountains and the ,quitman ,mountains in ,hudspeth ,county1 and in the ,chinati ,mountains and the ,shafter area in ,presidio ,county4 ,most of the mining has been from the ,shafter area 7this area is described with silver minerals on p4 #ij74 ,in central ,texas1 several small galena deposits have been found in ,blanco1 ,burnet1 and other counties of the ,llano uplift area4 ,some galena has been mined at ,silver ,creek in northwestern ,burnet ,county4 ,here1 galena occurs in cracks and as scattered grains in ,cambrian limestones and sandstones4 ,it is probable that much of the galena in west ,texas and in central ,texas was formed when hot magma forced out solutions containing lead4 ,these solutions moved up through cracks and other openings in the subsurface rocks and deposited the galena in them4 ,small amounts of galena1 which likely had a different origin1 have been found in ,fisher1 ,foard1 ,hardeman1 and ,young counties4 ,a little occurs also in rocks associated with salt in a number of the ,gulf ,coastal ,plain salt domes4 ,garnet ,garnet is not one mineral but is the name given to a group of several minerals that are very much alike4 ,in fact1 it often is impossible to tell some of them apart without using special laboratory tests4 ,illustration3 ,garnet crystal forms include3 ,a1 trapezohedron2 ,b1 dodecahedron2 ,c and ,d1 combination trapezohedron and dodecahedron4 ,the garnet minerals have glassy to resinous lusters and are transparent or translucent4 ,a pocket knife will not scratch them1 and some specimens are too hard even for quartz to scratch4 ,two of the garnet minerals most commonly found in ,texas are almandite1 an iron-aluminum silicate1 and grossularite1 a calcium-aluminium silicate4 ,almandite has a deep-red or a brownish-red color4 ,grossularite is pale green1 brownish yellow1 cinnamon brown1 or rose red4 ,garnet minerals occur as crystals and as masses that are scattered through some of the metamorphic and igneous rocks4 ,after they have weathered out of these rocks1 the garnets make up a part of many sands and sandstones4 ,because these minerals so commonly occur as crystals1 it is helpful to learn to recognize the crystal shapes4 ,garnet minerals are found in the igneous and metamorphic rocks of both central ,texas and west ,texas4 ,in central ,texas1 they occur in ancient ,precambrian schist and pegmatite rocks of the ,llano uplift area4 ,some of these central ,texas garnet localities are in northeastern ,mason ,county1 central and northwestern ,llano ,county1 west-central ,burnet ,county1 and northeastern ,gillespie ,county4 ,in west ,texas1 garnets occur in metamorphic rocks in the ,quitman ,mountains1 which are southwest of ,sierra ,blanca in ,hudspeth ,county1 and in the ,mica ,mine area1 which is south of ,van ,horn near the ,hudspeth-,culberson ,county line4 ,garnets also have been found in igneous rocks in the ,franklin ,mountains a few miles north of ,el ,paso in ,el ,paso ,county4 ,garnets that are found in metamorphic rocks such as schists were formed when great forces squeezed and heated rocks far below the earth's surface4 ,this heat and pressure caused elements in the rocks to join together into different combinations to form new minerals1 such as garnets4 ,garnets that occur scattered through igneous rocks1 such as some pegmatites and granites1 cooled and crystallized from hot1 igneous fluids when the rocks themselves formed4 ,most ,texas garnets are not transparent4 ,a few1 however1 are clear enough to be used as gemstones4 ,these can be cut1 polished1 and mounted in rings1 brooches1 bracelets1 and earrings4 ,although some garnet is widely used as an abrasive1 none from ,texas has been produced for this purpose4 ,gneiss ,gneiss is a metamorphic rock that has parallel layers or bands4 ,some gneiss is made up of the same minerals 7chiefly feldspar and quartz7 as granite1 and it is then called granite gneiss4 ,several of the other kinds of gneiss are known as mica gneiss1 conglomerate gneiss1 gabbro gneiss1 and hornblende gneiss4 ,in order to be a gneiss1 a metamorphic rock has to have bands or layers4 ,these bands may be either straight or wavy and either wide or narrow4 ,in most gneisses1 you will find a layer made up of long or flat mineral grains next to a layer made up of the grains of an entirely different mineral4 ,the bands may show color differences1 too4 ,for example1 a pink layer made up of feldspar grains may be found next to a black layer made up of hornblende grains4 ,the mineral grains interlock as they do in igneous rocks1 and they are generally large enough to be seen without a magnifying glass4 ,illustration3 ,gneiss from ,blanco ,county1 ,texas1 showing light and dark bands4 ,gneiss can form from an igneous rock1 such as granite1 or from a sedimentary rock1 such as sandstone4 ,heat1 fluids1 and pressures below the earth's surface change these rocks into gneiss4 ,gneiss that formed during ,precambrian time is now seen at the surface in both west ,texas and central ,texas4 ,in west ,texas1 it occurs principally in the ,van ,horn area of ,culberson and ,hudspeth counties4 ,in central ,texas1 it is found in ,blanco1 ,burnet1 ,gillespie1 ,llano1 and ,mason counties of the ,llano uplift area4 ,one of the ,llano uplift rocks is called the ,valley ,spring ,gneiss4 ,it generally has a light color 7much of it is pinkish71 and it is believed to have once been a sandstone4 ,another gneiss of this area1 the ,big ,branch ,gneiss1 which has a medium to dark gray color1 occurs in northern ,gillespie and ,blanco counties and is an altered igneous rock4 ,some of the ,texas gneiss rocks are suitable for use as building stones4 ,gold ,gold commonly occurs in nature as a single element--gold--but much native gold has a small amount of some other element1 such as silver1 copper1 or iron1 mixed with it4 ,native gold is a shiny1 yellow1 metallic mineral that does not tarnish1 and it leaves a shiny1 golden-yellow streak when you rub it across a streak plate4 ,if silver is present1 the color and streak have a lighter shade4 ,pure gold is extremely heavy--its specific gravity is #ai4c4 ,because it is malleable1 this mineral will flatten into a thin sheet when hammered4 ,it is ductile enough to be drawn out into wires4 ,gold is also soft--a pocket knife will scratch it easily4 ,when it is to be used for ornaments and jewelry1 gold is usually mixed with other metal1 such as silver1 copper1 nickel1 or palladium1 to make it harder4 ,the amount of gold that is present is then indicated by carats 7or karats74 ,pure gold is #bd carats4 ,if you have a gold ring that has #ad ,k stamped inside it1 you know that it is made of a mixture of #ad parts gold and #aj parts of other metal4 ,gold commonly occurs in nature as plates1 scales1 or grains4 ,some of the grains are large enough to be called nuggets4 ,it also is found in a wire-like shape described as filiform1 it occurs in a network1 called reticulate1 and it can have a branching and fern-like shape1 described as dendritic4 ,gold is not often found as individual crystals4 ,several other minerals1 such as pyrite1 chalcopyrite1 and mica1 are sometimes mistaken for gold4 ,none of these1 however1 is malleable and ductile1 and none is nearly as heavy as gold4 ,pyrite and chalcopyrite have dark-colored streaks unlike that of gold4 ,mica cleaves so perfectly that it can be split into thin1 flat sheets1 but gold has no cleavage at all4 ,the best places to look for gold are in areas near igneous rocks and along the creeks and rivers that drain these areas4 ,it is thought that most gold originally was carried up from molten igneous rock by hot solutions4 ,the solutions moved into cracks and other openings in nearby rocks and deposited the gold1 commonly along with quartz4 ,later1 some of these gold-bearing rocks weathered away4 ,the gold that the rocks contained either remained at the spot or was washed into creeks and rivers4 ,these transported accumulations of loose gold are called placer deposits4 ,illustration3 ,placer gold in very small quantities has been found in some of the stream gravels of ,texas4 ,no really important gold deposit has ever been found in ,texas1 although traces and small amounts have been reported in several areas4 ,a little gold has been found in the ,llano uplift area of central ,texas4 ,it occurs in quartz veinlets that cut through some of the ,precambrian metamorphic rocks of ,llano1 ,mason1 northeastern ,gillespie1 and west-central ,burnet counties4 ,many years ago1 a small amount of gold was mined northeast of ,llano in ,llano ,county from the ,heath mine4 ,some gold also has been found in sands and gravels along streams1 such as along ,sandy ,creek and its tributaries1 in parts of this ,llano uplift area4 ,in the ,trans-,pecos country of west ,texas1 small amounts of gold have been found in the ,van ,horn area of ,culberson and ,hudspeth counties1 in the ,quitman ,mountains district of ,hudspeth ,county1 and in the country around ,shafter in ,presidio ,county4 ,most of the small quantity of gold that was mined in west ,texas was obtained as a by-product from the ,presidio mine in the ,shafter district 7described with silver minerals on p4 #ij74 ,small amounts of gold have been reported from other parts of ,texas4 ,some of these localities are in ,eocene ,tertiary sandstones in the ,gulf ,coastal ,plain1 in ,cretaceous limestones in ,irion1 ,uvalde1 and ,williamson counties1 and in sand and gravel in ,howard and ,taylor counties4 ,none of these deposits has been found to have any commercial value4 ,granite ,granite is an intrusive igneous rock that is made up chiefly of crystalline grains or crystals of quartz and a feldspar mineral1 such as orthoclase or microcline4 ,several other minerals1 including mica and hornblende1 may also be present4 ,all of the mineral grains in granite are about the same size1 and you can distinguish them without using a magnifying glass4 ,a granite may be coarse grained1 medium grained1 or fine grained4 ,when you examine this rock1 you will see that its grains are not cemented but interlocked like the pieces of a jigsaw puzzle4 ,the color of granite1 which is pink1 red1 gray1 or brownish1 depends chiefly on the color of its feldspar grains4 ,most granites formed from hot1 molten magma that slowly cooled and hardened far below the earth's surface4 ,because of this slow cooling1 fairly large mineral grains were formed4 ,illustration3 ,polished section of pink granite from ,gillespie ,county1 ,texas4 ,granites are now seen at the surface in several areas of ,texas4 ,they were gradually uncovered as the areas became higher and the overlying rocks slowly weathered away4 ,one of these areas is the ,llano uplift of central ,texas where the granites occur in ,blanco1 ,burnet1 ,gillespie1 ,llano1 and ,mason counties4 ,these granites formed during ,precambrian time and are believed to be about a billion years old4 7,scientists are now able to determine the age of some rocks accurately by very precisely measuring the relative amounts of isotopes produced by decay of radioactive minerals47 ,illustration3 ,texas ,state ,capitol building at ,austin is made of ,burnet ,county granite obtained from ,granite ,mountain near ,marble ,falls1 ,texas4 ,granites also appear at the surface in the ,trans-,pecos country of west ,texas4 ,some of these areas include the ,franklin ,mountains of ,el ,paso ,county1 the ,quitman ,mountains of ,hudspeth ,county1 the ,chisos ,mountains of ,brewster ,county1 and the ,chinati ,mountains of ,presidio ,county4 ,red1 pink1 and gray granites from quarries in the ,llano uplift area are widely used as building stones and monument stones4 ,a large quarry at ,granite ,mountain just west of ,marble ,falls in ,burnet ,county has supplied pink granite for buildings in many parts of the ,united ,states4 ,the ,texas ,capitol building and several other ,state buildings in ,austin are made of this granite4 ,graphite ,graphite is a mineral that is made up of a single element--carbon4 7,diamond1 although it does not look at all like graphite1 is a crystalline form of carbon47 ,graphite is a steel-gray or black mineral that commonly has a metallic luster4 ,it is not heavy and is extremely soft4 ,graphite will soil your fingers and leave a black mark on paper4 ,this mineral cleaves perfectly in one direction and splits into thin flakes that feel greasy4 ,to help distinguish graphite from molybdenite1 a mineral it resembles1 you can use a shiny1 glazed surface1 such as is found on a saucer or a plate1 to test its streak4 ,when rubbed across this kind of surface1 graphite will leave a black streak1 but molybdenite will leave a greenish one4 ,graphite commonly occurs as scales1 as sheet-like layers1 or as compact masses4 ,it may be found mixed with clay or other impurities1 and it then looks dull and earthy4 ,crystals of graphite1 which are seldom found1 are #f-sided and flat4 ,graphite occurs in ,llano1 ,burnet1 and other counties in the ,llano uplift area of central ,texas4 ,one of the ,nation's most important graphite mines is located in the ,clear ,creek area several miles northwest of ,burnet in ,burnet ,county4 ,some graphite has also been mined near ,lone ,grove in ,llano ,county4 ,in addition1 a graphite schist1 obtained south of ,llano in ,llano ,county1 has been used as a filtering material4 ,illustration3 ,graphite is used in pencil lead1 generator brushes1 and lubricants4 ,all of this graphite occurs in extremely old ,precambrian graphite schist rocks that we now see at the surface in this part of ,texas4 ,it is believed that the schists were once ancient sedimentary rocks1 such as shales1 which contained organic matter4 ,long ago1 great forces below the earth's surface altered these rocks4 ,when this happened1 the organic material that they contained changed into the mineral we know as graphite4 ,graphite has a number of uses4 ,it is mixed with clay to make the pencil lead that we use for writing4 ,it serves as a lubricant1 either alone or mixed with oil1 grease1 or water4 ,in addition1 graphite is used to make generator brushes1 stove and shoe polish1 and special paints4 ,because it can stand great heat without melting1 some graphite is mixed with clay to make the pots or crucibles that hold molten metals4 ,grossularite4 ,see ,garnet4 ,gypsite4 ,see ,gypsum4 ,gypsum ,gypsum is a hydrous calcium sulfate4 ,this mineral is normally colorless or white1 but impurities cause it to appear gray1 brownish1 yellowish1 or reddish4 ,it is transparent or translucent and is not heavy4 ,when you rub gypsum across a streak plate1 it leaves a white streak4 ,this mineral is so soft that a fingernail scratches it easily4 ,gypsum occurs in several varieties4 ,the colorless1 glassy1 and transparent variety of gypsum is called selenite4 ,it is found as cleavable masses and as crystals that are prism-shaped or flat and diamond-shaped4 ,it is not uncommon for two crystals to be joined together so that they have a swallow-tail shape--these crystals are twinned4 ,groups of flat selenite crystals arranged together so that they resemble flowers are called rosettes4 ,many of these have been found in ,nolan ,county4 ,gypsum has four directions of cleavage4 ,one of these directions is so perfect that some selenite splits into thin1 clear sheets that may be mistaken for mica2 other selenite cleavage fragments may be mistaken for calcite4 ,you can distinguish selenite sheets from calcite by testing their hardness 7selenite is softer7 and by putting a drop or two of dilute hydrochloric acid on them4 ,the acid will fizz and bubble on calcite but not on the selenite gypsum4 ,there is also a quick way to distinguish the thin selenite cleavage fragments from mica4 ,after you carefully bend a thin sheet of mica1 it will snap back to its original shape without breaking4 ,selenite gypsum1 however1 is not elastic4 ,it will bend1 but it will break if you try to straighten it again4 ,illustration3 ,selenite gypsum crystal from ,bastrop ,county1 ,texas4 ,selenite is found in cracks and cavities in rocks4 ,good crystals have been collected at ,gyp ,hill1 a salt dome southeast of ,falfurrias in ,brooks ,county1 and some selenite has been mined there4 ,selenite crystals also occur scattered through clays1 particularly along creek banks1 in ,lee1 ,fayette1 ,bastrop1 and several other counties4 ,another variety of gypsum is known as fibrous gypsum4 ,it is made up of slender1 brittle1 needle-like fibers that fill the cracks in some rocks4 ,if fibrous gypsum has a silky or pearly luster1 it is called satin spar4 ,one of the places where satin spar occurs is in ,permian rocks in ,hardeman ,county4 ,illustration3 ,selenite gypsum rosettes from ,nolan ,county1 ,texas4 ,most of the fibrous gypsum and selenite is formed by solutions4 ,some of these solutions develop when underground waters1 seeping through rocks1 pick up and dissolve minerals that contain sulfur 7such as pyrite74 ,this dissolved material changes the water into very weak sulfuric acid4 ,when the sulfuric acid meets calcium carbonate 7as in limestone or calcite71 it combines with the calcium to form the gypsum4 ,illustration3 ,fibrous gypsum from ,terlingua area1 ,brewster ,county1 ,texas4 ,a massive1 fine-grained1 and translucent variety of gypsum1 known as alabaster1 is used for articles such as lamp bases1 statuettes1 vases1 and book-ends4 ,a loose1 earthy1 crumbly variety of gypsum1 called gypsite1 is ordinarily found mixed with other materials1 such as clay1 sand1 and soil4 ,it occurs either at or near the surface of the ground4 ,gypsite is found in ,culberson1 ,reeves1 and other counties in west ,texas4 ,a massive1 granular variety of gypsum1 called rock gypsum1 may occur in large deposits4 ,this is the gypsum that is used for making products such as plaster1 wallboard1 and some cements4 ,deposits of rock gypsum are found both underground and at the surface in ,texas4 ,surface deposits occur in ,permian rocks in several counties to the east of the ,texas ,high ,plains4 ,they also occur in the area between the ,pecos ,river and the ,delaware and ,apache ,mountains in ,culberson and ,reeves counties4 ,some of the other surface deposits are found near the ,malone ,mountains in ,hudspeth ,county and in ,lower ,cretaceous rocks in ,gillespie and ,menard counties4 ,rock gypsum has been mined from the deposits in ,fisher1 ,gillespie1 ,hardeman1 ,hudspeth1 and ,nolan counties4 ,it also has been produced from the cap-rock at ,hockley salt dome in ,harris ,county4 ,gypsum and another mineral1 anhydrite1 have very nearly the same composition4 ,both are calcium sulfates4 ,gypsum1 however1 contains water of crystallization1 and anhydrite does not4 ,it is likely that most of the rock-gypsum deposits of ,texas originally were beds of anhydrite4 ,by absorbing water that seeped through it1 the anhydrite changed into gypsum4 ,halite ,halite1 sodium chloride1 is the table salt you sprinkle on food for seasoning4 ,this mineral ordinarily is white or colorless1 but other materials cause it to be tinted red1 blue1 gray1 brown1 or green4 ,when you rub halite across a streak plate1 it leaves a white streak4 ,because halite cleaves in three directions1 all at right angles to each other1 the cleavage fragments are shaped like cubes4 ,you can see some of them by looking at a few grains of table salt through a magnifying glass4 ,halite has a salty taste and dissolves easily in water4 ,it also is transparent to translucent and has a glassy luster4 ,this mineral is soft enough for a copper penny to scratch it4 ,halite commonly occurs as cubic crystals and as granular or compact masses4 ,in addition to its use as table salt1 much halite goes to make soda ash1 chlorine1 and other chemicals4 ,a few of its other uses are in leather making1 meat packing1 and food canning4 ,texas has large underground deposits of halite4 ,these deposits1 known as rock salt1 occur in the ,permian subsurface basin of west ,texas and in the salt domes of the ,gulf ,coastal ,plain4 ,the ,permian basin1 which extends under parts of west ,texas1 ,new ,mexico1 ,oklahoma1 ,colorado1 and ,kansas1 is now completely filled with sediments4 ,it appears level and flat when you travel across it and does not look at all like a basin or a valley4 ,during ,permian time1 however1 this area was covered by a salty sea4 ,as the sea gradually dried up1 the dissolved material that it contained was deposited as thick beds of halite1 anhydrite1 and other minerals4 ,later1 these minerals were covered by sedimentary rocks which were deposited on top of them4 ,now1 the minerals are found many hundreds of feet below the surface4 ,in ,hutchinson1 ,mitchell1 ,ward1 and ,yoakum counties1 some of this ,permian basin salt has been produced 7as brine7 from wells that have been drilled into it4 ,illustration3 ,salt domes1 which are huge1 underground columns of halite1 occur on the ,gulf ,coastal ,plain4 ,the ,gulf ,coastal ,plain salt domes are huge and almost circular columns of halite1 some of which are more than #b miles wide4 ,some are less than #cjj feet below the surface1 but most of them are much deeper4 ,these salt columns pushed upward many thousands of feet from great1 deeply buried salt deposits4 ,the halite is mined from shafts dug into the ,hockley salt dome in ,harris ,county and into the ,grand ,saline salt dome in ,van ,zandt ,county4 ,salt brines are produced from wells drilled into several salt domes of this area4 ,at the surface in ,texas1 halite occurs in salt lakes in ,crane and ,hudspeth counties and in alkali lakes on the ,high ,plains4 ,it is found also on the shores of bays and lagoons in ,cameron1 ,kenedy1 ,kleberg1 and ,willacy counties1 and it occurs at springs and seepages in various places in the ,state4 ,hematite ,hematite1 iron oxide1 the chief ore of iron1 is found in many places in ,texas but not in large deposits4 ,this mineral may have a metallic luster and appear reddish brown1 dark brown1 steel gray1 or black or it may occur as a soft1 red1 earth-like material called red ocher4 ,illustration3 ,specular hematite from ,carrizo ,mountains1 ,hudspeth ,county1 ,texas4 ,most metallic hematite is too hard for a pocket knife to scratch1 but quartz or a steel file will scratch it4 ,hematite is fairly heavy1 for it has a specific gravity of #e4bf4 ,this mineral has no cleavage1 but some specimens show three directions of parting that are almost at right angles to each other4 ,a great help in identifying hematite is the dark reddish-brown streak it leaves when you rub it across a streak plate4 ,some hematite occurs as rounded masses that resemble kidneys or bunches of grapes 7then called kidney ore72 it also is found as flat crystals4 ,most of the ,texas hematite occurs as granular or compact masses4 ,one of these massive varieties is composed of shiny scales or plates and is called micaceous or specular hematite4 ,this variety has been found in ,hudspeth ,county and in northeastern ,mason ,county4 ,hematite also commonly occurs as cementing material in many ,texas sandstones4 ,some hematite is formed by the alteration of magnetite1 another iron mineral4 ,this hematite is known as martite1 and some of it still has the crystal shape 7an octahedron or a dodecahedron7 that belonged to the magnetite4 ,most of the hematite found in the ,llano uplift area of central ,texas is believed to be altered magnetite4 ,in this central ,texas area1 some massive1 granular martite has been mined at the ,gamble prospect1 a few miles southeast of ,fredonia in northeastern ,mason ,county1 where it occurs as layers in ,precambrian gneiss4 ,small deposits of hematite occur in other parts of ,texas1 too4 ,some of the west ,texas localities include ,sierra ,blanca1 the ,quitman ,mountains1 and the ,carrizo ,mountains of ,hudspeth ,county and the area around ,shafter in ,presidio ,county4 ,hollandite4 ,see ,manganese ,minerals4 ,hyalite4 ,see ,opal4 ,jasper4 ,see ,quartz4 ,kaolin4 ,see ,clay4 ,limestone ,limestone is a sedimentary rock made up chiefly of calcite1 a calcium-carbonate mineral4 ,this rock also commonly contains grains of quartz1 clay minerals1 the mineral dolomite1 or other materials4 ,if a large amount of dolomite is present1 the rock is called dolomitic limestone4 ,in some limestones1 the mineral grains are too small to be distinguished from each other without a magnifying glass or a microscope1 but in other limestones1 the individual mineral grains are easily seen4 ,pure limestone is white1 but if it contains clay or plant or animal matter it is light gray1 dark gray1 or black4 ,limestone also may be some shade of yellow1 brown1 or red4 ,it is fairly soft and can be scratched with a knife4 ,because this rock contains calcite1 an easy chemical test will help identify it3 a drop or two of dilute hydrochloric acid will quickly fizz and bubble when placed on the limestone4 ,limestones form in fresh water1 such as in lakes1 but most of them form in the seas4 ,as some earlier-formed rocks are weathered1 the calcium minerals that they contain are dissolved4 ,creeks and rivers carry this dissolved material to the sea4 ,there1 small animals1 such as corals1 crinoids1 sponges1 and foraminifers1 take the dissolved material out of the water to build their calcium carbonate shells4 ,plants1 such as algae1 can take calcium carbonate out of solution too1 and it collects on them4 ,shells1 shell fragments1 and plant remains accumulate on the sea floor1 forming limy deposits that later become limestone4 ,limestones also originate in a slightly different way4 ,when the temperature and chemical composition of the water permit1 calcium carbonate precipitates as millions of tiny grains of calcite and forms a limy mud that is converted to limestone4 ,many limestones contain shell or plant fragments in addition to these tiny grains of calcite4 ,illustration3 ,polished section of ,lower ,cretaceous ,edwards ,limestone from ,travis ,county1 ,texas1 containing fossil gastropods4 ,there are several special kinds of limestone4 ,if the rock is made up of many little rounded calcite grains that resemble fish eggs1 it is called olitic limestone4 ,another limestone1 chalk1 is soft1 white1 and fine grained4 ,it consists mostly of tiny shell fragments and fine-grained calcite4 ,coquina is a porous limestone made up of loosely cemented shells and shell fragments4 ,another special kind of limestone1 known as lithographic limestone1 because it can be used in printing1 is smooth1 firm1 and hard4 ,its mineral grains are too small to be recognized without a microscope4 ,this kind of limestone breaks with a smooth1 sometimes curved1 fracture4 ,still another variety1 pulverulent limestone1 is loose1 soft1 powdery1 and white4 ,it occurs in the ,lower ,cretaceous ,edwards ,limestone in ,williamson and ,bell counties of central ,texas4 ,some of this limestone is used to polish rice grains1 and it is added to livestock feeds to provide calcium for the animals4 ,illustration3 ,limestone quarry in ,lower ,cretaceous ,edwards ,limestone at ,georgetown1 ,williamson ,county1 ,texas4 ,much limestone is found at the surface in ,texas in ,cambrian1 ,ordovician1 ,mississippian1 ,pennsylvanian1 ,permian1 and ,cretaceous formations4 ,if you will look at numbers #e1 #f1 #i1 #aj1 and #aa on the ,texas geologic map 7pp4 #d-e71 you will see that these strata appear at the surface in central1 north-central1 and ,trans-,pecos ,texas4 ,limestone has many important uses4 ,much ,texas limestone is crushed and used as a road-building material and as an aggregate that is mixed with cement to make concrete4 ,farmers in some areas improve their crops by adding limestone to the soil4 ,limestone also is sent to the iron furnaces in east ,texas to be used in the production of pig iron and steel4 ,some of the ,texas limestones are heated to a fairly high temperature in order to change them into lime 7calcium oxide74 ,industry uses a large amount of lime in making chemicals1 steel1 glass1 paper1 and other products4 ,builders use it to make plasters1 mortars1 and stuccos4 ,at plants in ,comal1 ,johnson1 ,travis1 and ,williamson counties1 lime is made from ,cretaceous limestones4 ,another important use of limestone is in making portland cement4 ,the limestone is mixed with clay or shale1 and the mixture is burned in a kiln until it just begins to melt4 ,then it is allowed to cool4 ,next1 it is finely ground and in order to keep the finished cement from hardening or setting too quickly when it is used1 a retarder1 such as gypsum1 is added4 ,a number of cement-manufacturing plants in ,texas use ,cretaceous limestones1 shales1 and clays4 ,many of the ,texas limestones make excellent building stones4 ,some of them are quarried from ,pennsylvanian and ,cretaceous formations in north-central ,texas and from ,lower ,cretaceous formations in counties near the ,llano uplift of central ,texas4 ,a large quarry on the ,williamson-,travis ,county line near ,cedar ,park in central ,texas has supplied ,cretaceous limestone for many buildings and monuments in the ,united ,states and ,canada4 ,limonite ,limonite is not really a definite mineral but is a mixture of iron oxides containing water4 ,it is believed to be closely related to an iron mineral called goethite4 ,some limonite may be dull and earthy with the appearance of brownish-yellow or rusty brown clay4 ,this variety is so soft that a fingernail will scratch it easily4 ,other limonite has a dark brown or black color and a metallic or almost metallic luster4 ,a copper penny will not scratch it1 but a steel file will4 ,this kind of limonite may have a shiny black surface that resembles glossy lacquer4 ,the property that will help you most in identifying limonite is the rusty1 yellowish-brown streak it leaves when rubbed across a streak plate4 ,limonite has no cleavage and no crystal shape of its own4 ,but crystals of other iron minerals1 such as pyrite and magnetite1 alter to form limonite4 ,it then occurs with a crystal shape that originally belonged to one of these other minerals4 7,such false forms of minerals are called pseudomorphs47 ,limonite also occurs as layers in rocks1 as hollow or solid concretions1 or as coatings on other minerals4 ,it is found mixed with minerals such as clays and serves as the cementing material in some sandstones4 ,limonite is found in many localities in ,texas including ,blanco1 ,brewster1 ,burnet1 ,llano1 and ,san ,saba counties4 ,the most important limonite deposits in ,texas1 however1 are in the eastern part of the ,state1 particularly in ,anderson1 ,cass1 ,cherokee1 ,henderson1 ,marion1 ,morris1 ,nacogdoches1 ,smith1 and ,upshur counties4 ,the east ,texas limonite deposits occur mainly in ,weches sedimentary rocks4 ,these rocks1 which were deposited in the sea during ,eocene ,tertiary time1 contain clay along with greensands4 7,greensands are small1 soft grains that contain glauconite1 a mineral composed of iron1 silicon1 and several other elements47 ,later1 as the sea retreated1 these sediments became a part of the land4 ,waters seeping through the sediments changed into weak solutions of carbonic and sulfuric acid that dissolved the iron out of some of the greensands4 ,when conditions were favorable1 this iron was re-deposited as an iron-carbonate mineral called siderite4 ,siderite was changed to limonite by weathering4 ,some siderite is still found in east ,texas1 and it is also mined along with the limonite as an iron ore4 ,east ,texas iron ore has been mined from time to time ever since about #ahee1 and records show that a number of local iron furnaces once operated4 ,the brown iron ore 7as the limonite is also called7 now is mined from open pits in ,cass1 ,cherokee1 and ,morris counties4 ,this ore1 after being washed1 goes into blast furnaces at ,lone ,star 7near ,daingerfield7 and at ,houston4 ,in the blast furnaces the ore is changed into metallic iron by mixing it with coke 7made from coal7 and limestone and blowing in blasts of hot air4 ,to make steel1 the iron from the blast furnace 7called pig iron7 is put into open-hearth furnaces together with scrap iron1 limestone1 and other materials4 ,this mixture is heated and melted together to get rid of unwanted substances4 ,then other elements1 such as molybdenum1 manganese1 or nickel1 are added to make steel with the right strength and toughness4 ,illustration3 ,limonite ore is changed to metallic iron in a blast furnace4 ,skip car ,hot ,gases ,blast ,furnace ,iron ,ore ,limestone ,coke ,fire ,brick lining ,steel ,plate covering #ceej;,f ,hot ,air ,blast ,slag ,slag ,ladle ,molten ,iron ,iron ,ladle ,steel mills alongside the furnaces in ,texas turn out many products1 such as steel plates for oil tanks1 ships1 and tank cars and steel beams for framework in buildings and bridges4 ,some of their other products include pipes for the oil and chemical industries and wire for nails and fencing material4 ,lithographic ,limestone4 ,see ,limestone4 ,llanite ,llanite is a unique rock that is found only in ,llano ,county in central ,texas4 ,this intrusive igneous rock is made up of easily seen crystals and grains of quartz and feldspar that are scattered through a brown-colored mass of extremely small mineral grains4 ,the quartz is beautiful1 sky-blue1 and opal-like2 the feldspar has a rusty pink color4 7,because the quartz looks like opal1 this rock often is called opaline granite47 ,the mineral grains that make up the brown-colored mass are so tiny that they can be identified only with a microscope4 ,they are quartz1 feldspar1 mica1 fluorite1 and apatite4 ,llanite formed during ,precambrian time4 ,molten rock material forced its way upward into cracks that cut across granite and schist rocks while the rocks were still far underground4 ,this hot magma remained in the cracks where it cooled and hardened to form long1 narrow1 wall-like masses 7called dikes7 of llanite4 ,we can see some of the llanite dikes exposed at the earth's surface to the north and northeast of ,llano in ,llano ,county because the overlying rocks have weathered away4 ,llanite has been quarried from one of the dikes west of ,babyhead in northern ,llano ,county4 ,because llanite is both attractive and strong1 it has been used as an ornamental stone and as a monument stone4 ,magnetite ,magnetite1 iron oxide1 is a black1 metallic mineral with an outstanding physical property3 it is magnetic--fragments of magnetite readily cling to a magnet4 ,it also leaves a black streak when rubbed across a streak plate4 ,although this mineral is too hard to be scratched by the average pocket knife1 a steel file will scratch it4 ,magnetite is fairly heavy--it has a specific gravity of #e4ah4 ,magnetite occurs as compact or granular masses1 as scattered grains1 and as crystals4 ,most of the crystals are octahedrons1 but some dodecahedrons are found4 ,magnetite helps make up a part of many metamorphic and igneous rocks1 and it also occurs as tiny crystals and grains in some sands1 sandstones1 and other sedimentary rocks4 ,illustration3 ,metallic iron1 after leaving the blast furnace1 is made into steel in an open-hearth furnace4 ,open-,hearth ,furnace ,scrap metal ,alloying ,elements ,limestone ,furnace ,interior ,live ,fuel ,burner ,air pre-heated ,most of the magnetite that has been found in ,texas occurs in ,precambrian gneiss and schist rocks of the ,llano uplift area of central ,texas1 particularly in ,llano ,county and in eastern ,mason ,county4 ,it occurs as thin layers1 as thick lens-shaped deposits1 and as scattered grains in the rocks4 ,probably at least a billion years ago these gneisses and schists were sedimentary rocks1 such as shales and sandstones4 ,some geologists believe that these rocks could have contained iron sediments 7perhaps in the form of glauconite74 ,great forces below the earth's surface crumpled and squeezed the sedimentary rocks and changed them into the metamorphic schist and gneiss rocks we see today4 ,as this happened1 the iron sediments in the rocks were changed into magnetite4 ,illustration3 ,granular magnetite fragments from northwest of ,llano1 ,llano ,county1 ,texas1 are attracted to a magnet4 ,at least some of the magnetite in this area 7such as the deposit at ,iron ,mountain in ,llano ,county7 probably had a different sort of origin4 ,molten igneous rock material containing iron could have moved up into cracks in the ancient sedimentary rocks4 ,then the magnetite formed from this iron material when the igneous and sedimentary rocks were changed into the schists and gneisses of today4 ,none of the ,llano and ,mason ,county magnetite deposits is really very large4 ,nevertheless1 prospecting and a little mining have been carried on from time to time at several deposits in this area4 ,at ,iron ,mountain1 which is about #ab miles northwest of ,llano in ,llano ,county1 magnetite has been mined from open pits4 ,although magnetite is commonly used as a source of iron1 the magnetite from this deposit was used as a heavy concrete aggregate4 ,malachite4 ,see ,copper ,minerals4 ,manganese ,minerals 7,braunite1 ,hollandite1 ,pyrolusite7 ,although manganese does not occur alone in nature as a native element1 it makes up a part of many minerals and compounds4 ,this element has an important use in steel making1 where it helps rid the steel of unwanted substances1 such as oxygen and sulfur1 and1 in addition1 it is used to make tough1 hard1 manganese steel for armor plate1 railroad tracks1 safes1 and steam shovels4 ,manganese has various uses outside the steel industry4 ,it is added to copper and nickel to make alloys1 it is used in the manufacture of dry-cell batteries1 and 7as manganese sulfate7 it is used as a fertilizer4 ,manganese minerals and compounds1 such as braunite1 hollandite1 pyrolusite1 and wad1 occur in several counties in ,texas4 ,no large1 commercial deposits have been found here4 ,some manganese compounds and minerals are covered with a soft1 sooty black material that will soil your fingers4 ,this can help you recognize these minerals2 however1 a few non-manganese minerals1 such as some chalcocite1 also have a black coating that soils your fingers in a similar way4 ,one of the manganese minerals1 braunite1 is a complex oxide of manganese that contains silica4 ,it has a submetallic luster and is dark steel-gray or black4 ,when rubbed across a streak plate1 it leaves a steel-gray or a black streak4 ,this mineral is too hard to be scratched by a pocket knife1 but a piece of quartz or a steel file will scratch it4 ,braunite has a specific gravity of #d4ge to #d4hb4 ,it has four directions of cleavage that are parallel to the faces of a pyramid4 ,in the ,spiller mine1 about #ae miles northeast of ,mason in ,mason ,county1 masses of braunite occur as lens-shaped layers in ,precambrian gneiss and quartzite rocks4 ,this braunite may have formed from another manganese mineral 7possibly manganese garnet7 that was exposed at the earth's surface after the overlying rocks eroded away4 ,as this other mineral weathered1 it may have altered into braunite1 or the braunite could have been deposited from solutions emanating from hot magmas before the great thickness of overlying rock was removed4 ,the mineral variety hollandite is a rare manganate of manganese and barium4 ,it has a metallic luster1 and its color is silvery gray or black4 ,when you rub it across a streak plate1 hollandite leaves a black streak4 ,it has a specific gravity of #d4g to #e4 ,hollandite is rather hard1 but a steel file will scratch it4 ,illustration3 ,hollandite from ,jeff ,davis ,county1 ,texas4 ,hollandite occurs in western ,jeff ,davis ,county in west ,texas at what is called the ,mayfield prospect4 ,here1 it is found as rounded masses that occur in a vein near a large fault in ,lower ,cretaceous limestone rocks4 ,other manganese compounds1 pyrolusite and wad1 are found in several important deposits near the ,pecos ,river in western ,val ,verde ,county4 ,pyrolusite is a manganese dioxide mineral4 ,it is black1 opaque1 and so soft that it rubs off on your fingers like soot4 ,pyrolusite may be granular and massive or may be powdery4 ,it also occurs as a fern-like coating on rocks4 ,wad is not really a mineral but is an impure1 dull-black or brownish-black mixture of manganese oxide1 water1 and other substances4 ,it can be soft enough to soil your fingers1 or it can be too hard to scratch with a pocket knife4 ,wad occurs in earthy or compact masses or in crusts or stains on rocks4 ,in ,val ,verde ,county1 the wad and pyrolusite are found mixed with soil1 clay1 gravel1 sand1 and plant remains4 ,this material fills cracks in ,lower ,cretaceous limestones1 it is scattered through gravels1 and it is deposited in low places at the surface4 ,the manganese in these deposits came from limestone rocks that have since weathered away4 ,rainwater trickled into these rocks and dissolved the manganese minerals they contained4 ,this manganese was washed down toward the ,pecos ,river and was deposited as wad and pyrolusite4 ,marble ,marble is a metamorphic rock made up chiefly of sparkling grains of calcite or dolomite1 but other minerals may be present4 ,the marble may be fine grained1 medium grained1 or coarse grained2 commonly1 all the mineral grains are about the same size4 ,marble may be of uniform color1 banded1 spotted1 or streaked4 ,if it is made up only of pure calcite or dolomite1 the marble is white4 ,if1 however1 it contains carbonaceous material1 such as graphite1 it is grayish or black4 ,limonite impurities cause the marble to be yellowish brown1 and manganese oxides and hematite give it a brownish1 pinkish1 or reddish color4 ,marble is a rather soft rock1 and you can scratch it easily with a pocket knife4 ,a few drops of dilute hydrochloric acid will bubble and fizz readily on calcite marble2 on dolomite marble1 it may fizz slightly4 ,marble forms from limestone or from dolomite rock4 ,heat and pressure below the earth's surface cause the calcite and dolomite mineral grains in these rocks to recrystallize4 ,a fine-grained limestone can be changed into a coarse-grained calcite marble4 ,the marble is not made up of new and different minerals1 but it has a new texture unlike that of the limestone4 7,to a builder1 the word marble has another meaning4 ,he considers rocks such as unaltered limestone1 unaltered dolomite1 or even serpentine to be marble1 if they will take a high polish47 ,illustration3 ,polished section of ,precambrian metamorphic marble from ,llano ,county1 ,texas4 ,metamorphic marbles occur at the surface in central ,texas and in west ,texas4 ,some of the west ,texas occurrences are in the ,van ,horn area of ,culberson and ,hudspeth counties and in the ,big ,bend area of ,brewster ,county4 ,in central ,texas1 ,precambrian marbles are found in ,burnet1 ,gillespie1 ,llano1 and ,mason counties of the ,llano uplift area4 ,many of them are suitable for use as monument and building stones4 ,some of the ,llano ,county marble is quarried and used as granules for roofs and as terrazzo chips for making colorful floors 7described with serpentine on p4 #hh74 ,martite4 ,see ,hematite4 ,mica ,mica is not just one mineral but is the name given to a group of similar minerals4 ,the mica minerals are easy to recognize4 ,because they have perfect cleavage in one direction1 they split into thin1 flat sheets4 ,you can see through some mica sheets1 and they are elastic enough to be bent back and forth4 7,another mineral1 selenite gypsum1 also will split into thin1 flat1 transparent sheets1 but selenite sheets break when you bend them47 ,illustration3 ,mica minerals have perfect cleavage in one direction1 resulting in thin1 sheet-like cleavage fragments4 ,basal ,cleavage ,two of the mica minerals that you are most likely to find in ,texas are muscovite and biotite4 ,both these minerals are potassium-aluminum silicates1 and biotite1 in addition1 contains magnesium and iron4 ,in general1 muscovite is light colored1 that is1 it has a light brown1 yellow1 or green tint1 or is colorless1 and biotite is dark colored1 commonly dark green1 brown1 or black4 ,these minerals have glassy or pearly lusters and are rather soft--a copper penny scratches them4 ,the specific gravity of biotite is #b4h to #c4b1 and that of muscovite is #b4gf to #c4a4 ,mica minerals occur in igneous rocks1 such as granite and pegmatite1 and in metamorphic rocks1 such as schist and gneiss4 ,they also are found as tiny flakes in some sandstones1 limestones1 and other sedimentary rocks4 ,most of the ,texas mica is found in the ,llano uplift area 7particularly in ,llano ,county7 and in the ,mica ,mine area4 7,the ,mica ,mine area is in the ,van ,horn ,mountains about #ae miles south of ,van ,horn in west ,texas47 ,in both these areas1 the mica minerals occur mostly in ,precambrian pegmatites and mica schists4 ,the gleaming mica schists were once igneous rocks or sedimentary rocks such as sandstones and shales4 ,long ago1 great forces beneath the earth's surface changed the rocks into mica schists4 ,the mica that is found in pegmatites formed from hot fluids of igneous origin when the pegmatite rock itself was formed4 ,clusters of mica in the pegmatites are called books1 because the thin sheets into which the mica splits look like pages4 ,some muscovite books up to #h inches across are found in the ,mica ,mine area of the ,van ,horn ,mountains4 ,the books or sheets of muscovite mica that occur in pegmatites are especially valuable to industry4 ,muscovite can stand great heat without melting1 it is tough1 it splits into thin sheets1 and it lets very little heat and electricity pass through4 ,because of these properties1 muscovite is used in fuses and as insulators in heating elements of electric irons and toasters4 7,biotite is not used1 because the iron it contains makes it a conductor of electricity47 ,sheet muscovite also is widely used by the electronics industry as a non-conducting material in the manufacture of tubes and other products4 ,both muscovite and biotite from mica schist rocks1 as well as scrap pieces of sheet mica from pegmatites1 are ground into flakes or powder4 ,this ground-up mica has many uses1 ranging from a powder coating for automobile inner-tubes to ,christmas tree 8snow40 ,only a small amount of mica has been mined in ,texas4 ,a fair grade of sheet mica occurs in the pegmatites at ,mica ,mine in west ,texas1 but the deposit is not large4 ,in the pegmatites of the ,llano uplift area of central ,texas1 no sheet mica has been found that is considered good enough for the requirements of industry4 ,mica suitable for grinding1 however1 is found in both these ,texas areas4 ,micaceous ,hematite4 ,see ,hematite4 ,microcline4 ,see ,feldspar4 ,milky ,quartz4 ,see ,quartz4 ,muscovite4 ,see ,mica4 ,native ,silver4 ,see ,silver ,minerals4 ,obsidian and ,vitrophyre ,obsidian is a dark1 glassy-looking igneous rock4 ,most obsidian contains the same chemical elements as granite and rhyolite1 since all three of these rocks can form from the same type of molten rock material4 ,obsidian1 however1 has no separate minerals1 because its chemical elements are not combined in an orderly way4 ,it is a natural glass4 ,because it is a glass1 we know that obsidian forms very quickly4 ,one way for it to form is from the sudden cooling of hot1 molten lava that flows out of volcanoes4 ,if the lava cools and hardens before the separate minerals can crystallize1 it becomes a natural glass1 such as obsidian4 ,this rock is smooth and shiny4 ,most of it is black1 but some can be dark green or dark brown4 ,obsidian allows light to pass through it1 and it breaks with a curved1 conchoidal fracture4 ,the broken edges are very sharp4 ,another glassy igneous rock that forms from fast-cooling lava is vitrophyre4 ,it looks like obsidian except that it has crystals or crystalline mineral grains 7which may be light colored7 scattered through the dark glassy material4 ,illustration3 ,obsidian was used by the ,indians to make arrowheads4 ,obsidian and vitrophyre are found in the ,big ,bend area of ,brewster and ,presidio counties in west ,texas4 ,they occur with other igneous rocks that formed there during ,tertiary time4 ,the ,indians who long ago roamed this area used the smooth1 shiny vitrophyre and obsidian to make some of their arrowheads and scrapers4 ,today1 rock collectors pick up these attractive rocks for their collections1 and some of them cut and polish obsidian and vitrophyre for use as gemstones4 ,onyx4 ,see ,quartz4 olitic ,limestone4 ,see ,limestone4 ,opal ,opal is like hardened jelly or gelatin4 ,it has no crystalline inner structure and no crystal shape of its own--it is amorphous4 ,this mineral has almost the same chemical composition as quartz4 ,both are silicon dioxides 7silica71 but opal1 in addition1 contains water4 ,opal can be almost any color--red1 yellow1 blue1 brown1 gray1 white--or it can be colorless4 ,it is transparent or translucent and appears glassy1 resinous1 greasy1 or dull4 ,opal has a specific gravity of #a4i to #b4b--this mineral is a little lighter than quartz4 ,it also is softer than quartz4 ,a copper penny will not scratch opal1 but quartz will4 ,opal has a white streak and a curved1 conchoidal fracture but no cleavage4 ,opal occurs in a number of places in ,texas4 ,in the ,trans-,pecos country of west ,texas1 it fills cracks and cavities in some of the extrusive igneous rocks4 ,it occurs on the ,high ,plains of northwest ,texas1 and it is found in ,tertiary formations of the ,gulf ,coastal ,plain where it occurs as masses that fill cracks and cavities in sedimentary rocks1 as the cementing material in some sandstones 7such as in the ,catahoula sandstone71 and as opalized wood4 ,much opal forms from underground waters that contain silicon4 ,these solutions move through the rocks and deposit the opal in them4 ,opal is found in a number of varieties4 ,some show a beautiful1 lustrous play of colors that comes from inside the specimens4 ,these varieties are known as precious opal and are prized as gemstones4 ,in ,texas1 some precious opal is found near ,alpine in ,brewster ,county4 ,it has a milky white to bluish-white color1 is translucent1 and shows a fiery orange1 red1 blue1 and green play of colors4 ,the variety known as common opal shows no play of colors4 ,it may be white1 gray1 bluish1 reddish1 greenish1 or yellowish1 and it is only slightly translucent4 ,it is found in ,brewster1 ,jeff ,davis1 ,presidio1 and other counties of the ,trans-,pecos country of west ,texas4 ,it occurs also around some of the wet-weather 7playa7 lakes on the ,texas ,high ,plains4 ,in the ,gulf ,coastal ,plain1 common opal is found with chalcedony 7a variety of quartz7 in ,tertiary formations4 ,a south ,texas locality sometimes visited by collectors is near ,freer in ,duval ,county4 ,a clear1 commonly rounded1 variety of opal that looks like ice is called hyalite4 ,two areas in which it has been found are in ,presidio ,county in west ,texas and in ,llano ,county in central ,texas4 ,illustration3 ,opalized wood from ,washington ,county1 ,texas4 ,a variety of petrified wood1 called opalized wood1 is opal that replaced the fibers of a piece of wood4 ,wood opal is found at a number of places in the ,gulf ,coastal ,plain4 ,it occurs there in ,tertiary formations within about #bj miles of the boundary line between areas #b and #c shown on the geologic map 7pp4 #d-e74 ,a soft opaline material called diatomite1 or diatomaceous earth1 is made up chiefly of the skeletons of diatoms--tiny1 one-celled plants that live in fresh or salt water4 ,these little plants are able to take silica from the water to make opal skeletons for themselves4 ,when the diatom skeletons collect at the bottom of a lake or sea1 they form the light1 crumbly1 white1 gray1 or cream-colored deposit of impure opal known as diatomite4 ,industry uses this material as a filter1 as insulation1 as an abrasive1 and as a filler4 ,diatomite formed in ancient lakes on the ,texas ,high ,plains during late ,tertiary 7,pliocene7 and early ,quaternary 7,pleistocene7 times4 ,it is found in ,armstrong1 ,crosby1 ,dickens1 ,ector1 ,hartley1 and ,lamb counties4 ,opaline ,granite4 ,see ,llanite4 ,orthoclase4 ,see ,feldspar4 ,pegmatite ,pegmatites occur in igneous rock areas1 and most geologists consider them intrusive igneous rocks4 ,they are made up of crystals and crystalline mineral grains that fit together--the grains are interlocked4 ,the crystals and grains in pegmatites are larger than those of surrounding rocks1 and some are huge1 even larger than a man4 ,however1 there is a wide range of grain sizes in pegmatite4 ,some pegmatites cut through igneous or metamorphic rocks in such a way that they resemble walls 7called dikes74 ,others are found as veins1 as flat masses1 or as odd-shaped bodies in rocks4 ,many pegmatites occur in granites and contain feldspar1 quartz1 mica1 and other minerals1 as granite does4 ,some pegmatites occur with other kinds of igneous rocks and contain the same minerals as these rocks4 ,a few pegmatites contain rare and unusual minerals4 ,many geologists believe that pegmatites form from hot fluids of igneous origin that are left after other igneous rocks1 such as granite1 have already formed4 ,these left-over fluids contain large amounts of aluminum1 potassium1 silicon1 sodium1 and several other elements4 ,while the granite or other rocks are still far underground1 this material pushes up into them1 and may even partly dissolve them4 ,then it slowly cools and hardens into pegmatite4 ,it is believed that1 later1 more fluids move into cracks in some pegmatites4 ,this new material adds other minerals to the pegmatites and alters some of those minerals already there4 ,illustration3 ,quartz-feldspar pegmatite from ,burnet ,county1 ,texas4 ,some of the pegmatites we now see at the surface in ,texas are probably about a billion years old4 ,they formed during ,precambrian time and occur with other extremely old rocks4 ,one well-known ,texas pegmatite area is the ,mica ,mine district of west ,texas4 ,it is about #ae miles south of ,van ,horn in the ,van ,horn ,mountains of ,culberson and ,hudspeth counties4 ,another pegmatite area is in the ,llano uplift of central ,texas4 ,these central ,texas pegmatites occur in ,burnet1 ,gillespie1 ,llano1 and ,mason counties4 ,large crystals and grains of feldspar1 mica1 and quartz are found in the pegmatites of both these areas4 ,a small amount of mica has been mined from the west ,texas pegmatites1 and feldspar has been produced from the central ,texas pegmatites4 ,an extremely rare and unusual pegmatite occurs in the ,llano uplift area at ,baringer ,hill1 which is west of ,burnet in ,llano ,county4 ,this pegmatite was once on the bank of the ,colorado ,river1 but when ,buchanan ,dam was built1 the area was flooded4 ,the ,baringer ,hill pegmatite now lies beneath the water of ,lake ,buchanan4 ,many rare minerals1 which contain beryllium1 cerium1 thorium1 uranium1 yttrium1 zirconium1 and a number of other elements1 occur in this pegmatite4 ,some of these minerals1 such as those containing yttrium and zirconium1 glow or incandesce when they are heated4 ,during the early part of this century1 before the area was flooded1 several of the yttrium minerals were mined and used in making lamp mantles4 ,pitchblende4 ,see ,uranium ,minerals4 ,precious ,opal4 ,see ,opal4 ,pulverulent ,limestone4 ,see ,limestone4 ,pumicite4 ,see ,volcanic ,ash4 ,pyrite ,pyrite is a shiny1 pale golden-yellow or brassy-yellow metallic mineral4 ,this mineral1 an iron disulfide1 is so often mistaken for gold that it is widely known by the nickname fool's gold4 ,except for their similar color and luster1 pyrite and gold are really very different4 ,when you rub pyrite across a streak plate1 it leaves a black1 a greenish-black1 or a brownish-black streak1 but the streak of gold is gold-colored4 ,pyrite is too hard for the average pocket knife to scratch1 but a knife will scratch gold easily4 ,pyrite is brittle and readily breaks1 but gold is malleable and flattens out when hit with a hammer4 ,pyrite is only about #e times as heavy as an equal volume of water1 but pure gold is over #ai times as heavy4 ,and pyrite may have a brown or a multicolored tarnish on it1 but gold never tarnishes4 ,illustration3 ,pyrite veins in white marble from ,llano ,county1 ,texas4 ,illustration3 ,cubic crystals of pyrite4 ,pyrite is a common mineral and is found in many of the igneous1 metamorphic1 and sedimentary rocks of ,texas4 ,it may be scattered through the rocks1 or it may fill cracks and cavities in them4 ,this mineral occurs as granular and compact masses1 as rounded masses1 or as crystals4 ,the crystals are commonly cubes1 pyritohedrons1 or octahedrons4 ,in some crystals1 the shapes are combined 7such as a cube with an octahedron or two pyritohedrons grown through each other74 ,you may notice that the sides of some cubes and pyritohedrons have fine1 parallel grooves 7called striae or striations7 on them4 ,pyrite originates in a number of different ways4 ,some of it forms1 along with other minerals in igneous rocks1 from hot magmas4 ,it also forms in metamorphic rocks by the same processes that produce these rocks4 ,some of the pyrite in limestone and other sedimentary rocks is formed when the rocks themselves are deposited by seas or streams4 ,pyrite also is deposited by the hot fluids that are given off by magmas4 ,these fluids travel up into cracks and other openings in rocks and then form pyrite as well as other minerals4 ,much pyrite forms in still another way4 ,as water seeps through rocks1 it dissolves some of the iron minerals that they contain4 ,when1 under certain conditions1 these iron solutions mix with hydrogen sulfide 7this is the gas that makes some water smell like rotten eggs71 pyrite is formed4 ,pyrite alters easily4 ,because of this1 most builders carefully check the limestone1 granite1 marble1 or whatever other building stone they plan to use to be sure that it does not contain large amounts of pyrite4 ,when exposed to the weather1 pyrite changes to limonite and causes an unsightly rust stain4 ,pyrite is used as a source of sulfur1 and it is produced for this purpose in several states4 ,in ,texas1 however1 no pyrite deposits have been found that are large enough to be mined4 ,pyrolusite4 ,see ,manganese ,minerals4 ,quartz ,quartz1 silicon dioxide1 is one of the most common minerals4 ,it is glassy1 waxy1 greasy1 or dull and is transparent or translucent4 ,pure quartz is colorless1 but impurities make some varieties white1 black1 or a shade of red1 yellow1 blue1 violet1 or brown4 ,quartz is a hard mineral4 ,it scratches window glass and cannot be scratched by a pocket knife or even by a steel file4 ,it has a specific gravity of #b4fe4 ,the curved1 conchoidal fracture shown by many specimens helps identify it4 ,quartz is plentiful in ,texas4 ,it occurs in igneous rocks1 such as granite1 llanite1 and pegmatite2 in metamorphic rocks1 such as quartzite1 schist1 and gneiss2 and in sedimentary rocks1 such as some sandstone1 conglomerate1 and breccia4 ,illustration3 ,quartz crystal1 with inclusions1 from ,burnet ,county1 ,texas4 ,quartz is found as crystals and as masses4 ,some of the masses are coarsely crystalline1 but some are made up of extremely small crystalline particles called cryptocrystalline quartz4 ,some of the cryptocrystalline varieties of quartz found in ,texas are chalcedony1 chert1 and jasper4 ,some of the coarsely crystalline varieties found here are amethyst1 milky quartz1 rose quartz1 smoky quartz1 and rock crystal4 ,illustration3 ,amethyst geode from the ,alpine area of ,brewster ,county1 ,texas4 ,a colorless1 glassy variety of quartz1 called rock crystal1 is clear enough to see through4 ,it is found as crystals that are #f-sided prisms with pyramid-like faces on the ends4 ,this variety is commonly associated with igneous rocks1 such as those of the ,llano uplift area of central ,texas and of the ,trans-,pecos country of west ,texas4 ,it is commonly used as a gemstone and is made into necklaces1 earrings1 and other jewelry4 ,some specimens of rock crystal have slender1 needle-like crystals of other minerals1 such as tourmaline1 actinolite1 or rutile1 enclosed in them4 ,a clear1 glassy variety of quartz1 amethyst1 has a purple or violet color4 ,it1 like rock crystal1 is commonly found in #f-sided prisms with pyramid-shaped ends and is also prized as a gemstone4 ,amethyst has been found in ,precambrian rocks in the ,llano uplift area of central ,texas4 7,amethyst ,hill1 a locality well known to collectors for many years1 is in northeastern ,gillespie ,county47 ,in west ,texas1 amethyst has been found in ,cenozoic igneous rocks in the ,sierra ,blanca and ,quitman ,mountains of ,hudspeth ,county and in the ,alpine area of ,brewster ,county4 ,illustration3 ,milky quartz from ,burnet ,county1 ,texas4 ,a variety of quartz with a milk-white color and a glassy to greasy luster is called milky quartz4 ,it occurs either as crystals or as crystalline masses4 ,very little light will pass through it4 ,in central ,texas1 milky quartz occurs abundantly in the ,precambrian rocks of the ,llano uplift area in ,blanco1 ,burnet1 ,gillespie1 ,llano1 and ,mason counties4 ,it also is found in some of the rocks of the ,trans-,pecos country of west ,texas1 such as in the ,carrizo ,mountains of ,culberson and ,hudspeth counties4 ,other good places to look for this variety of quartz are in the sands and gravels along many streams in ,texas4 ,some quartz has a glassy to a greasy luster and a rose or pink color4 ,rose quartz1 as this variety is called1 commonly occurs as masses rather than as individual crystals4 ,it can be found along some of the streams in ,texas and also in igneous rocks1 such as those of the ,llano uplift area of central ,texas4 ,a kind of quartz with a smoky brown1 a smoky yellow1 or a dark brownish-black color is called smoky quartz4 ,its luster is glassy1 and it may be either translucent or transparent4 ,smoky quartz is commonly found as crystals that are shaped like #f-sided prisms with pyramid-like ends4 ,it is commonly associated with igneous rocks1 and beautiful specimens have been found in the ,lake ,buchanan area of ,llano and ,burnet counties in central ,texas4 ,illustration3 ,smoky-quartz crystals from ,burnet ,county1 ,texas4 ,a cryptocrystalline variety of quartz1 chalcedony1 has a waxy to dull luster and a tan1 white1 gray1 or light-blue color4 ,it is translucent but not transparent4 ,chalcedony does not have its own crystal shape but instead is found in masses that line or fill cracks1 pores1 and other cavities in rocks4 ,it is formed when water containing silicon slowly seeps into these openings in the rocks and deposits the silicon dioxide there as chalcedony4 ,chalcedony commonly occurs in some of the ,tertiary rocks of the ,gulf ,coastal ,plain4 ,for example1 chalcedony associated with opal is found near ,freer in northern ,duval ,county4 ,in the ,high ,plains of west ,texas1 it is found in alkali-lake deposits1 such as at ,shafter ,lake in ,andrews ,county and at ,cedar ,lake in ,gaines ,county4 ,in the ,trans-,pecos country of west ,texas1 it can be found filling small cavities in extrusive igneous rocks4 ,illustration3 ,polished agate from ,rio ,grande gravels of ,zapata ,county1 ,texas4 ,a variety of chalcedony that generally is made up of more than one color is called agate 7although agates consisting of several shades of a single color are also found74 ,the colors may be spread out unevenly so that the agate has a cloudy appearance1 or they can be arranged in wavy1 in straight1 or in concentric lines or bands4 ,if the bands are straight and parallel1 the specimen is called onyx4 ,agate that has a moss-like or tree-like design in it is called moss agate4 ,some agates make attractive gemstones when cut and polished4 ,illustration3 ,jasper from ,uvalde ,county1 ,texas4 ,dark areas are brownish red2 light areas are a yellowish-tan4 ,much agate has been found filling cavities in ,cenozoic igneous rocks in ,brewster1 ,presidio1 and other counties in the ,trans-,pecos country of west ,texas4 ,it has been found also in an area about #aj to #ae miles wide along the ,rio ,grande1 mostly in southern ,webb ,county and in ,zapata and ,starr counties4 ,trees and other plants have been replaced by agate4 ,many specimens of agatized wood have been collected from ,tertiary formations in ,fayette1 ,gonzales1 ,lee1 ,washington1 and other counties of the ,gulf ,coastal ,plain4 7,the agatized wood1 along with opalized wood1 occurs within about #bj miles of the boundary between no4 #b and no4 #c on the geologic map1 pp4 #d-e47 ,a hard1 compact1 slightly translucent variety of cryptocrystalline quartz is called jasper4 ,it commonly has a red1 brown1 or yellow color due to the presence of an iron oxide1 such as hematite4 ,some jasper is made up of irregular bands of more than one of these colors4 ,this variety of quartz often is polished to make attractive gem or ornamental stones4 ,it has been collected at several localities in ,texas1 particularly from creek and river gravels4 ,starr and other nearby counties along the ,rio ,grande have furnished a number of good specimens4 ,a hard1 smooth1 compact1 translucent rock that is made up mostly of cryptocrystalline quartz is called chert or flint4 ,it is white1 black1 or some shade of gray1 brown1 or pink1 and its luster is waxy1 slightly glassy1 or dull4 ,chert is found in many creek and river gravels in ,texas4 ,it also occurs with limestone1 such as in the ,lower ,cretaceous ,edwards ,limestone of central ,texas and in the ,ordovician ,ellenburger strata in the ,llano uplift area4 ,chert also is found with the ,ordovician rocks of the ,marathon area of ,brewster ,county4 ,geologists do not agree on whether chert and flint are two names for one variety of rock1 or whether each is a separate variety4 ,some1 however1 now give chert a geological meaning and flint an archaeological meaning4 ,they use the word chert to describe geological formations or rock specimens4 ,they give the name flint to the same rock when it has been used by ,indians in making arrowheads1 scribers1 scrapers1 and spearheads4 ,quartzite ,quartzite is either a metamorphic rock or a sedimentary rock4 7,the sedimentary kind of quartzite is described with sand and sandstone on p4 #hf47 ,metamorphic quartzite is made up mostly of quartz4 ,it forms when heat and fluids below the earth's surface cause the grains and cement of a quartz sandstone to recrystallize4 ,when this happens1 the grains interlock and are no longer held together by cement4 ,metamorphic quartzite1 like sedimentary quartzite1 is a hard1 firm rock that breaks through the quartz grains instead of between them4 ,ancient ,precambrian metamorphic quartzite occurs at the surface in the ,llano uplift area of central ,texas1 in the ,van ,horn area of west ,texas1 and in the ,franklin ,mountains north of ,el ,paso in extreme west ,texas4 ,rhyolite ,rhyolite is a fine-grained or glassy igneous rock that commonly is extrusive or volcanic4 ,it has a pink1 red1 tan1 white1 gray1 purple1 or black color4 ,this rock1 like granite1 is made up chiefly of feldspar and a silica mineral1 such as quartz1 but other minerals may be present4 ,both rhyolite and granite form from the same kind of molten rock material4 ,nevertheless1 even though their compositions are the same1 these two rocks do not look alike4 ,their textures differ because granite forms slowly and rhyolite forms quickly4 ,much of the ,texas rhyolite formed from hot1 molten lava4 ,this lava flowed out onto the surface either through volcanic cones or cracks in the ground4 ,some of the lava cooled and hardened too quickly for mineral grains to develop4 ,this rapidly cooled lava formed a rhyolite rock that is made up1 at least partly1 of glass4 ,in many of the rhyolites1 crystalline mineral grains were able to form1 but these grains are extremely small1 and you may not be able to distinguish them even with a magnifying glass4 ,some rhyolite1 because it hardened from moving1 flowing lava1 has streaks and bands of different colors and textures4 ,this rhyolite has flow structure4 ,one variety of rhyolite has easily seen crystals and grains of minerals1 such as feldspar1 quartz1 and mica1 scattered through a mass of the tiny crystalline grains 7in much the same way that raisins are scattered through a cake74 ,the easily seen crystals and grains are called phenocrysts1 and the rock itself is called a rhyolite porphyry4 ,many rhyolites and rhyolite porphyries occur in the ,tertiary igneous rocks of the ,trans-,pecos country of west ,texas4 ,just a few of these localities include the ,barrilla ,mountains of ,jeff ,davis and ,reeves counties1 the ,chisos ,mountains of ,brewster ,county1 the ,chinati ,mountains of ,presidio ,county1 and the ,davis ,mountains of ,jeff ,davis ,county4 ,rock ,crystal4 ,see ,quartz4 ,rock ,gypsum4 ,see ,gypsum4 ,rock ,salt4 ,see ,halite4 ,rose ,quartz4 ,see ,quartz4 ,salt4 ,see ,halite4 ,sand and ,sandstone ,sand is a loose1 uncemented sedimentary deposit made up of fragments of weathered rocks and minerals4 ,these fragments must be of a certain size 7between _/ millimeter and #b millimeters in diameter7 in order to be called sand grains4 ,the largest sand grains are about the size of a pinhead4 ,sand grains are smaller than the fragments known as granules2 they are larger than those known as silt4 ,many sands are made up chiefly of grains of quartz4 ,this mineral is plentiful and does not easily weather away4 ,in addition1 rock fragments and many other minerals1 such as feldspar1 mica1 gypsum1 magnetite1 and garnet1 are found as sand grains4 ,rains wash many of the sand grains and other weathered rock and mineral fragments into creeks and rivers4 ,these streams may carry the sand and other sediments long distances before depositing them4 ,today1 we find sands along the banks of many creeks and rivers in ,texas and along the beaches of the ,gulf of ,mexico4 ,the sand in the rivers is in transit to the ,gulf4 ,in addition1 sand occurs at the surface in other ,cenozoic formations and in some of the ,paleozoic and ,mesozoic formations of ,texas4 ,sand has many uses4 ,much building sand1 which is used in mortar and concrete1 is produced from numerous sand and gravel pits in ,texas4 ,pure quartz sand that can be used to make glass is known as glass sand4 ,some of it is found in north-central ,texas in ,lower ,cretaceous formations4 ,a large glass sand quarry is located at ,santa ,anna in ,coleman ,county4 ,along the ,gulf ,coastal ,plain1 sand that is used in glassmaking occurs in ,eocene ,tertiary strata4 ,a coarse-grained sand1 blast sand1 is used with compressed air to clean the walls of brick and stone buildings and to carve designs on monument stones1 such as marbles4 ,some coarse sand is also used as a filtering sand in purifying water4 ,these types of sand have been produced from the ,gulf ,coastal ,plain as well as from other areas of ,texas4 ,sand grains1 when nature cements them together1 make up the sedimentary rock sandstone4 ,some sandstones form when underground water carrying dissolved mineral matter moves through loose sand4 ,as the dissolved mineral matter comes out of solution1 it forms a cement that binds the sand grains together4 ,the cement may be material such as calcite 7calcium carbonate71 quartz1 chalcedony1 or opal1 which are silica minerals1 and limonite and hematite1 which are iron oxides4 ,clay also may serve as a cement4 ,it is either deposited along with the sand or is formed from weathered feldspar sand grains4 ,the color of the cementing material helps determine the color of the rock4 ,iron oxide cement1 for example1 causes the sandstone to have a reddish1 yellowish1 or brownish color4 ,sandstones also are white1 black1 gray1 green1 or cream colored4 ,ordinarily1 sandstones break through the cementing material1 not through the sand grains4 ,thus1 the broken surface of the rock feels rough and gritty4 ,some quartz sand grains1 however1 are tightly cemented with silica to form an extremely hard and compact rock4 ,if this rock breaks smoothly through the grains instead of between them1 it is known as quartzite4 ,some of this sedimentary quartzite occurs in the ,texas ,gulf ,coastal ,plain in the ,tertiary ,catahoula strata4 7,another kind of quartzite is described on pp4 #hd-he47 ,illustration3 ,sandstone from the ,eocene ,wilcox ,group of strata of northwestern ,zavala ,county1 ,texas4 ,ordinary sandstones are seen at the surface in many localities in ,texas1 and a number of them have been used as building stones4 ,some of the places where sandstones occur are in the ,cambrian and ,pennsylvanian formations of the ,llano uplift area of central ,texas and in the ,pennsylvanian1 ,permian1 and ,lower ,cretaceous formations of north-central ,texas4 ,tertiary sandstones occur in the ,texas ,gulf ,coastal ,plain1 and ,triassic sandstones are found along the edges of the ,texas ,high ,plains4 ,sandstone is also found in many formations of the ,trans-,pecos country of west ,texas4 ,sandstone4 ,see ,sand and ,sandstone4 ,satin ,spar4 ,see ,gypsum4 ,schist ,schist is a metamorphic rock that splits easily along thin1 generally parallel layers1 called folia4 ,these layers may be either straight or curved1 and they are made up of crystalline grains of one or more than one mineral4 ,this structure is called schistosity or foliation4 ,when you examine schist1 you will see that many of the mineral grains are flat or long1 and that they are lined up in one direction to form the layers4 ,some schists have fairly large crystals 7many with perfect shapes7 scattered through them4 ,for example1 mica schists may contain beautiful crystals of garnet4 ,each kind of schist is named for an outstanding mineral that it contains4 ,mica schist contains a large amount of mica4 ,we also find hornblende schist1 actinolite schist1 chlorite schist1 talc schist1 and graphite schist4 7,graphite schist is discussed with graphite on p4 #fc47 ,schists form from other rocks1 such as granite1 gabbro1 or shale4 ,the rocks are changed into schists by fluids and by heat and pressure below the earth's surface4 ,extremely ancient schists that formed during ,precambrian time are exposed at the surface in the ,allamoore--,van ,horn area of west ,texas and in the ,llano uplift area of central ,texas4 ,geologists believe that the ,packsaddle ,schist of the ,llano uplift area was once shale4 ,good exposures of this schist are seen in the ,honey ,creek area near ,packsaddle ,mountain in ,llano ,county4 ,schorl4 ,see ,tourmaline4 ,sedimentary ,quartzite4 ,see ,sand and ,sandstone4 ,selenite4 ,see ,gypsum4 ,serpentine ,serpentine is the name given both to a rock and to a mineral4 ,the mineral serpentine 7a hydrous magnesium silicate7 is found in two different forms4 ,if it is fibrous1 it is called chrysotile2 if it is layered and platy1 it is known as antigorite4 ,antigorite is brownish green and smooth and waxy looking4 ,some of it can be split into thin sheets4 ,chrysotile is made up of greenish1 silky fibers1 which may be brittle and break apart in large pieces4 ,if1 however1 the fibers can be pulled apart into soft flexible1 little threads1 the mineral is called chrysotile asbestos4 ,light will pass through both these varieties of serpentine1 and both are soft enough to be scratched by a pocket knife4 ,when rubbed across a streak plate1 they leave white streaks4 ,antigorite and chrysotile have no crystal shapes of their own1 but several other minerals can alter to form these two varieties of serpentine4 ,thus antigorite and chrysotile may be found as pseudomorphs in a crystal shape that originally belonged to another mineral4 ,antigorite and chrysotile are commonly found closely mixed with dolomite1 talc1 magnetite1 calcite1 pyrite1 and several other minerals4 ,these minerals make up serpentine rock 7also called serpentinite74 ,this rock ordinarily is some shade of green 7such as whitish1 yellowish1 brownish1 bluish1 or dark blackish green71 and it may be mottled4 ,it is brittle or tough and generally is massive4 ,serpentine rock1 like the serpentine minerals1 is fairly soft--you can scratch it with a pocket knife4 ,in the ,llano uplift area of central ,texas1 serpentine rock is found among ,precambrian metamorphic rocks1 such as gneiss and schist4 ,an especially large deposit in this area is known as the ,coal ,creek serpentine mass4 ,it is over #c miles long1 and at one place1 it is almost #a miles wide4 ,this mass of serpentine extends across the ,blanco-,gillespie ,county line in the extreme northern parts of these two counties4 7,a little fibrous chrysotile is found here1 but it will not break into flexible enough threads to be called chrysotile asbestos47 ,several other deposits of serpentine occur in northeastern ,gillespie ,county and in southern ,llano ,county4 ,it is believed that the ,coal ,creek serpentine was formed from an igneous rock such as peridotite1 which is made up chiefly of grains of the mineral olivine4 ,the peridotite may have been altered into serpentine by underground waters that seeped through it4 ,it is possible1 however1 that other serpentines in the area were formed when rocks were altered by hot fluids and great pressures far below the earth's surface4 ,the ,llano area serpentine has been widely used in terrazzo floors4 ,to make these floors1 small pieces of serpentine and other colored rocks are put into cement that is spread over a concrete slab4 ,then1 after the cement has hardened1 it is ground to a flat1 smooth surface and polished4 ,the resulting terrazzo floor is both colorful and durable4 ,serpentine rock also is cut into slabs1 polished1 and used as indoor building stones4 ,verde antique1 a variety often seen in the lobbies of office buildings1 consists of green serpentine rock with streaks of white calcite or dolomite in it4 ,in the ,balcones fault zone area 7shown on the ,texas physiographic outline map1 p4 #db7 from ,uvalde ,county to ,williamson ,county1 serpentine occurs with ,upper ,cretaceous rocks4 ,the serpentine rock is seen at the surface in a few places 7such as in ,travis and ,uvalde counties71 but much of it is underground4 ,in several oil fields of this area 7as at ,thrall field in ,williamson ,county and at ,lytton ,springs field in ,caldwell ,county71 the serpentine rocks contain oil4 ,serpentinite4 ,see ,serpentine4 ,shale ,shale is a sedimentary rock made up of tightly packed clay and mud particles4 ,it has a smooth appearance because it is so fine grained4 ,in fact1 most of the particles in it are too small to be distinguished with a magnifying glass4 ,these particles are the weathered remains of earlier rocks4 ,they were carried by creeks and rivers to other parts of the land or to the sea1 where they formed layers of clay and mud4 ,later1 other sediments were deposited on top of them4 ,the weight of these new sediments squeezed the clays and muds together to form firm1 compact shale4 ,shale looks very much like some clays4 ,it1 like clay1 can be almost any color4 ,if the shale contains animal or plant matter1 it is black1 gray1 or blue4 ,if it contains iron oxide 7many minerals containing iron alter to this material71 it is a shade of red1 yellow1 or brown4 ,shale is soft and can be easily scratched by a knife4 ,it also is brittle and crumbles easily4 ,this rock has a property that will help you to distinguish it from clay3 the particles that make up the shale were deposited in layers1 and the shale splits into flat1 thin flakes along these layers1 which clay will not do4 ,shale is fairly abundant in ,texas1 especially in ,mississippian1 ,pennsylvanian1 and ,cretaceous formations4 ,for example1 ,pennsylvanian shales are found at the surface in north-central ,texas1 in the area around the ,llano uplift of central ,texas1 and in the ,marathon and ,solitario uplifts of west ,texas4 ,many of shale's uses are the same as those of clay4 ,some of it can be used to make brick1 tile1 and other products1 and some is often used instead of clay in making portland cement4 ,cement plants at ,dallas1 ,el ,paso1 ,fort ,worth1 and ,waco are located at places where ,cretaceous limestones1 which also are used in cement making1 and ,cretaceous shales are found near each other at the surface4 ,oil shale1 from which petroleum can be obtained by heating1 has been found in central ,texas4 ,it occurs in ,mississippian formations in ,lampasas1 ,mc,culloch1 and ,san ,saba counties4 ,because oil is much less expensive to obtain from wells1 it is not produced from these shales4 ,silver ,minerals 7,argentite1 ,cerargyrite1 ,native ,silver7 ,silver has many uses4 ,like gold1 it is a beautiful metal that long has been used for coins and ornaments4 ,a large amount of silver goes to make articles such as spoons1 forks1 platters1 and trays4 ,the photographic industry uses silver--much of the film for cameras is coated with a silver halide4 ,doctors and dentists use silver1 too4 ,the mixture that a dentist uses to fill teeth contains silver along with several other metals4 ,doctors sometimes use silver wire to fasten broken bones1 and silver compounds and solutions1 such as silver nitrate1 are used in some kinds of medical treatment4 ,perhaps more people have heard of legendary1 lost silver mines of ,texas than of the actual and important silver deposits found in the ,trans-,pecos country of west ,texas4 ,some of the west ,texas silver minerals include argentite1 cerargyrite1 and native silver4 ,although the argentite and native silver commonly found there are mixed with galena1 a lead mineral1 or with chalcocite1 a copper mineral1 they also occur separately4 ,the element silver is found alone as native silver4 ,when pure1 it is rather easy to recognize4 ,it is metallic and has a silver-white color that may tarnish to gray1 black1 or yellowish brown4 ,native silver is heavy 7it has a specific gravity of #aj4e7 and soft 7a pocket knife scratches it easily74 ,when you rub it across a streak plate1 native silver1 unless it is tarnished1 leaves a shiny1 silver-white streak4 ,this metal is so ductile that it can be drawn into a wire4 ,it is also malleable and flattens when hit with a hammer4 ,silver occurs as crystals1 which are poorly shaped cubes and octahedrons1 or as irregular masses4 ,it may have a net-like appearance 7called reticulate71 or it may be shaped like little needles 7described then as acicular74 ,it occurs in wires 7then called filiform7 or as scales or plates4 ,illustration3 ,prospector4 ,two of the ,texas silver minerals1 argentite and cerargyrite1 do not resemble silver at all4 ,argentite1 a silver sulfide1 is also called silver glance4 ,it is a dark1 lead-gray mineral with a metallic luster that weathers to a dull black4 ,when you rub it across a streak plate1 argentite gives a shiny1 blackish to lead-gray streak4 ,this mineral is soft enough to leave a mark on paper4 ,it has a specific gravity of #g4c1 and it is sectile enough to be cut smoothly 7like soap7 with a knife4 ,in some places argentite is found as irregular masses or as a coating on rocks and other minerals4 ,another silver mineral1 cerargyrite 7or horn silver7 is a silver chloride4 ,this mineral has a nonmetallic luster and is transparent to translucent4 ,it resembles pearl-gray1 white1 greenish1 or colorless wax4 ,when exposed to the light it turns violet brown or black4 ,cerargyrite is soft--you can scratch it with a fingernail4 ,like argentite1 it is sectile4 ,this mineral has a specific gravity of #e4e1 and it commonly occurs as irregular masses and as crusts4 ,these silver minerals have been mined at a number of places in ,trans-,pecos ,texas4 ,the largest silver mine in ,texas1 the ,presidio mine1 is located near ,shafter in south-central ,presidio ,county4 ,it contains argentite1 cerargyrite1 and native silver1 along with galena and several other minerals4 ,this mine is not open now1 but in the years between #ahhe and #aidb1 it produced a large amount of silver along with some lead and gold4 ,there are several other lead-silver mines in this ,shafter area1 but none has produced as much as the ,presidio mine4 ,in this mine1 the silver minerals occur mostly in large1 flat deposits in ,permian limestone and other sedimentary rocks4 ,the minerals are believed to have been deposited there--probably during ,tertiary time--by solutions that came from hot magma far below the rocks4 ,as they moved in along the layers of limestone1 the solutions replaced portions of this rock with minerals containing silver1 lead1 and other elements4 ,later1 water seeped into these deposits and dissolved some of the minerals4 ,this dissolved material was then re-deposited1 and it formed most of the minerals we now find there4 ,no silver is being mined in ,texas at present1 but it has1 in the past1 been produced from other ,trans-,pecos mines4 ,galena that contains silver 7called argentiferous galena7 has been mined at the ,bird mine at ,altuda ,mountain 7about #ad miles east of ,alpine7 in northern ,brewster ,county4 ,it also has been obtained from mines in the ,quitman ,mountains and in the ,eagle ,mountains of ,hudspeth ,county4 ,some cerargyrite has been mined at the ,plata ,verde mine near the ,culberson-,hudspeth ,county line4 ,several mines in the ,van ,horn area of ,culberson and ,hudspeth counties have produced silver along with copper4 ,an important silver mine in this area is the now idle and flooded ,hazel mine4 7,this mine is described with copper minerals on p4 #eb47 ,smoky ,quartz4 ,see ,quartz4 ,soapstone4 ,see ,talc and ,soapstone4 ,specular ,hematite4 ,see ,hematite4 ,sulfur ,sulfur is one of ,texas' most valuable minerals4 ,it consists of only a single element1 sulfur4 ,this mineral has a resinous luster and is transparent to translucent4 ,sulfur ordinarily is yellow1 but impurities cause it to look greenish1 brownish1 reddish1 or grayish4 ,when you rub it across a streak plate1 it leaves a white or a pale-yellow streak4 ,sulfur has a specific gravity of #b4jd to #b4ji and is soft enough to be scratched by a copper penny4 ,it breaks with a conchoidal to uneven fracture4 ,when it gets hot enough 7#dgh ,fahrenheit71 sulfur will burn4 ,for this reason1 it often is called brimstone4 ,sulfur does not conduct electricity and is a poor conductor of heat4 ,you can test how poorly heat passes through it by holding a fragment of sulfur up to your ear4 ,you may be able to hear a crackling sound4 ,the sound results when the outer part of the fragment expands 7due to the heat from your hand7 while the inner part 7which has received no heat7 remains unchanged4 ,crystals of sulfur are sometimes found1 and most of them have either a double-pyramid shape or a flat1 tabular shape4 ,sulfur also occurs as compact masses1 as crusts1 and as scattered grains4 ,native sulfur deposits are found in two widely separated areas of ,texas--one in west ,texas and the other along the ,gulf ,coast in southeast ,texas1 extending over into ,louisiana4 ,in the ,gulf ,coast area1 native sulfur is found on some of the salt domes4 ,the salt domes are huge 7from about half a mile to more than #b miles across71 column-shaped masses made up of halite and some anhydrite4 ,these masses have pushed up toward the surface through thousands of feet of sand1 clay1 and other sedimentary rocks4 ,on top of many of the salt columns is a covering of limestone 7calcite71 anhydrite1 and gypsum known as the cap-rock4 ,it is in this cap-rock that the sulfur is found4 ,it is thought that when the masses of halite and anhydrite pushed toward the earth's surface1 some of the upper part of the halite dissolved4 ,the anhydrite1 however1 did not dissolve1 and it remained on top of the salt column4 ,then1 a part of this anhydrite was altered into the gypsum1 limestone1 and sulfur that now are found in some of the cap-rocks4 ,laboratory experiments have shown that the sulfur in the cap-rocks likely formed through the action of sulfate-reducing bacteria4 ,these bacteria1 in the presence of petroleum1 converted the sulfate in some of the anhydrite into hydrogen sulfide4 ,later1 hydrogen sulfide was oxidized--perhaps by reaction with more of the anhydrite--to form the sulfur4 ,most of the large cap-rock sulfur deposits are about #a1ejj to #b1djj feet underground4 ,at first1 an attempt was made to get this sulfur out of the ground by digging shafts down to it1 but loose1 wet1 caving sands and poisonous gases1 such as hydrogen sulfide1 made this mining method almost impossible4 ,finally1 a chemist1 ,herman ,frasch1 found a way to obtain the sulfur by making use of sulfur's low melting point4 ,when sulfur gets slightly hotter than boiling water 7#bce to #bdg ,fahrenheit71 it melts and becomes a dark1 yellowish-brown liquid4 ,in the ,frasch method of sulfur mining1 a well is drilled into the salt-dome cap-rock1 and three pipes1 one inside the other1 are put into the well4 ,superheated water under pressure 7hotter than #bab ,fahrenheit1 the temperature at which water ordinarily turns into steam7 is sent down one of the pipes to melt the sulfur in the cap-rock around the bottom of the well4 ,then1 compressed air is sent down another of the pipes4 ,this air presses against the liquid sulfur and forces it up to the surface through the third pipe4 ,at the surface1 the sulfur is poured into bins1 where it cools and becomes a solid again1 or it is transported molten1 in pipelines and tankers4 ,sulfur has been obtained from a number of the ,texas ,gulf ,coast salt domes including ,bryan ,mound1 ,clemens dome1 ,damon ,mound1 and ,hoskins ,mound in ,brazoria ,county2 ,palangana dome in ,duval ,county2 ,long ,point dome1 ,nash dome1 and ,orchard dome in ,fort ,bend ,county2 ,high ,island dome in ,galveston ,county2 ,fannett dome and ,spindletop dome in ,jefferson ,county2 ,moss ,bluff dome in ,liberty ,county2 ,gulf dome in ,matagorda ,county2 and ,boling dome in ,wharton ,county4 ,in west ,texas1 sulfur occurs in ,permian rocks both at the surface and underground4 ,a small amount of sulfur has been mined in the ,rustler ,springs area of northeastern ,culberson ,county and northwestern ,reeves ,county1 about #ej miles northwest of ,pecos4 ,there1 scattered grains1 crystals1 and irregular masses of sulfur occur in cracks and in dissolved-out openings in the ,castile ,gypsum and in the surface gravel1 gypsum1 sand1 and clay that cover most of this formation4 ,sulfur has many uses4 ,it is used as an insect-killer1 thus helping our food crops to grow4 ,it is used in pulp and paper manufacturing and in the vulcanizing of rubber4 ,some other uses are in the making of paints1 dyes1 and explosives4 ,a large amount of sulfur goes to make sulfuric acid1 which itself has numerous uses in the chemical1 steel1 oil refining1 and other industries4 ,illustration3 ,sulfur is obtained from the cap-rock of ,gulf ,coastal ,plain salt domes by the ,frasch process4 ,sulfur ,uncemented ,sediments ,limestone ,sulfur-,bearing ,limestone ,hot ,water ,melted ,sulfur ,anhydrite ,talc and ,soapstone ,talc1 a hydrous magnesium silicate1 is an extremely soft mineral--your fingernail scratches it easily4 ,it has a greasy or a pearly luster1 and its color is white1 light green1 or gray4 ,when rubbed across a streak plate1 it leaves a white streak4 ,talc cleaves perfectly in one direction1 and the cleavage fragments are thin1 flat1 and sheet-like4 ,its fracture is uneven4 ,this mineral has a soaplike or greasy feel1 and it is sectile--a knife will cut through it4 ,talc is not particularly heavy--it has a specific gravity of #b4g to #b4h4 ,this mineral seldom occurs with a crystal shape4 ,more commonly it is massive and is granular or layered4 ,talc is not always found as a single1 pure mineral4 ,in nature1 it commonly occurs mixed with one or more other minerals1 such as tremolite1 anthophyllite1 chlorite1 and magnetite4 ,this combination of talc with other minerals forms a soft1 greasy or soapy-feeling metamorphic rock called soapstone4 ,the talc in this rock may be difficult to identify without special laboratory tests4 ,in ,texas1 talc and soapstone are found in ,precambrian metamorphic rocks4 ,in west ,texas1 talc occurs in an area about #bj miles long 7just north of ,u4 ,s4 ,highway #hj in the vicinity of ,allamoore1 ,eagle ,flat siding1 and ,talc ,rock siding7 in ,hudspeth ,county4 ,some of this talc is mined from open pits and used by the ceramic industry to make wall tile4 ,some of it is finely ground1 mixed with insect poison1 and used as insect powders and dusts4 ,illustration3 ,talc schist from the ,allamoore area of ,hudspeth ,county1 ,texas4 ,deposits of soapstone1 containing talc1 occur in the ,llano uplift area of central ,texas with schist1 gneiss1 and serpentine rocks in northeastern ,gillespie1 northwestern ,blanco1 and southern ,llano counties4 ,smaller deposits occur in northeastern ,mason ,county and in northwestern and southeastern ,llano ,county4 ,the ,llano uplift area soapstones are light green to light buff4 ,it is thought that some of them were once igneous rocks that contained magnesium minerals4 ,fluids1 along with great heat and pressures below the earth's surface1 changed these igneous rocks into soapstone4 ,some of this ,llano uplift area soapstone is mined from open pits near ,willow ,city in ,gillespie ,county4 ,it is used mostly in making insect powders and roofing granules4 ,in addition1 some of the central ,texas soapstones have been used for hearths and for fireplace linings4 ,topaz ,topaz1 an aluminum fluorosilicate1 is a mineral especially prized by collectors because many specimens are gemstones4 ,topaz is transparent1 has a glassy luster1 and is quite hard 7neither quartz nor a steel file will scratch it74 ,the topaz that has been found in ,texas is either colorless1 pale blue1 or sky blue4 ,this mineral is fairly heavy--its specific gravity is #c4d to #c4f4 ,it cleaves perfectly in one direction 7called basal cleavage71 and some of the cleavage fragments have a flat1 slabby appearance4 ,topaz is commonly found as prism-shaped crystals1 as cleavage fragments1 and as irregular grains4 ,some fragments of topaz look like quartz4 ,topaz1 however1 is harder and heavier than quartz1 and it has perfect basal cleavage1 which quartz does not have4 ,in ,texas1 crystals1 grains1 and cleavage fragments of topaz occur in the ,llano uplift area of central ,texas4 ,they are found near ,streeter and ,grit in west-central ,mason ,county and near ,katemcy in northern ,mason ,county4 ,here1 some of the topaz occurs in ,precambrian pegmatite veins that cut through granite rocks4 ,most of the topaz1 however1 is found as pebbles in the gravels of nearby creeks1 where it has washed after weathering out of the rocks4 ,illustration3 ,topaz crystal from near ,streeter1 ,mason ,county1 ,texas4 ,topaz probably originates when hot fluids move up out of molten magma into cracks and cavities in the surrounding rocks4 ,there1 the fluids react with elements in the rocks to form the topaz4 ,topaz is a good gemstone because1 in addition to its beauty1 it is hard and is not easily marred by scratches4 ,the ,mason ,county topaz makes excellent gemstones4 ,most of it is beautiful and clear and is either colorless or of a pleasing blue color4 ,these stones are cut1 polished1 and mounted in rings and other jewelry4 ,a number of specimens of this ,mason ,county topaz are displayed in museums4 ,tourmaline ,tourmaline is a complex silicate of boron and aluminum4 ,other elements1 such as magnesium1 sodium1 lithium1 calcium1 iron1 or fluorine1 also may be present4 ,this mineral has a glassy to resinous luster4 ,only the dark-colored varieties of tourmaline have been found in ,texas4 ,one is a black variety called schorl1 and another is a brown variety called dravite4 ,other kinds of tourmaline1 although not found in ,texas1 are colorless or some shade of blue1 yellow1 red1 pink1 or green4 ,some crystals even show more than one color4 ,tourmaline is too hard to scratch with a steel file1 it has a specific gravity of #c to #c4be1 and it has a conchoidal to uneven fracture4 ,very little light passes through the dark varieties1 and some fragments of schorl look like shiny1 black coal4 ,tourmaline occurs as masses without crystal shapes1 but crystals are commonly found4 ,the crystals are prism-shaped and have small vertical grooves1 called striations1 on the prism faces4 ,when you look at some crystals from an end1 you will see that the cross section is a triangle with the sides bowed outward4 ,illustration3 ,black tourmaline crystals with milky quartz from north of ,llano1 ,llano ,county1 ,texas4 ,both the black and the brown varieties of tourmaline have been found at several places in the ,llano uplift of central ,texas4 ,one well-known locality is at ,town ,mountain north of ,llano in ,llano ,county4 ,here1 the tourmaline occurs in milky quartz that is associated with ,precambrian granite rocks4 ,in west ,texas1 in ,culberson and ,hudspeth counties1 black tourmaline occurs in pegmatite rocks in the ,van ,horn ,mountains1 the ,carrizo ,mountains1 and the ,wylie ,mountains4 ,in the ,eagle ,mountains of ,hudspeth ,county1 it is found in metamorphic rocks as well as in pegmatites4 ,some tourmaline formed from hot fluids containing boron that were given off by magmas far below the earth's surface4 ,these fluids traveled up through cracks and other openings in overlying rocks4 ,as the fluids reacted with other elements and compounds1 the tourmaline formed4 ,the clear1 light-colored varieties of tourmaline are much admired1 and they are more widely used as gemstones than are the dark-colored varieties4 ,some collectors1 however1 find that the dark-colored ,texas tourmalines1 when cut and polished1 make shiny1 attractive gemstones4 ,some tourmaline is used as grinding material1 but no ,texas tourmaline is produced for this purpose4 ,travertine4 ,see ,calcite4 ,uranium ,minerals 7,carnotite1 ,uranophane1 ,pitchblende7 ,in #aide1 the world suddenly became aware of the awesome power of atomic energy when the element uranium was used to produce some of the first atomic bombs4 ,uranium does not occur alone in nature but is found combined with other elements in a number of minerals4 ,all of the uranium minerals are radioactive4 ,the uranium they contain is gradually breaking down and changing into a series of #ac other elements1 called daughter elements4 ,each daughter element breaks down and changes into the next daughter element of the series4 ,while breaking down1 these elements give off particles and rays of energy4 ,this energy or radioactivity is made up of what are called alpha particles1 beta particles1 and gamma rays4 ,you cannot see1 hear1 taste1 smell1 or feel them4 ,the alpha and beta particles are weak and do not travel far4 ,the gamma rays1 however1 can travel farther and can pass through seemingly solid material4 ,scientists have found that these rays can move through about #a foot of rock1 #b feet of water1 and several hundred feet of air4 ,prospectors searching for uranium minerals carry instruments that are able to detect this radioactivity4 ,the uranium itself gives off only alpha particles1 but some of its daughter elements give off gamma rays4 ,these daughter elements are normally found with the uranium1 and it is their strong gamma rays that the instruments are most apt to detect4 ,illustration3 ,a ,geiger counter is used to detect radioactivity4 ,one of the instruments used is the ,geiger counter4 ,it indicates radioactivity by means of a meter1 a flashing light1 or a clicking sound1 which can be heard through earphones4 ,another instrument for detecting radioactivity is the scintillation counter4 ,it is more sensitive than the ,geiger counter and it can detect radioactivity from a greater distance4 ,the scintillation counter can be used from an automobile or an airplane1 but the ,geiger counter must be quite close to the source of radioactivity to be of use4 ,various uranium minerals have been found1 mostly in small amounts1 in a number of places in ,texas4 ,some of these minerals1 such as uraninite or pitchblende1 are heavy and dark colored4 ,others1 including carnotite1 tyuyamunite1 autunite1 and uranophane1 are a shade of yellow or green4 ,they are quite soft4 ,deposits of the light-colored uranium minerals have been mined from two areas of ,texas4 ,one of these areas is in ,garza ,county on the ,texas ,high ,plains1 and the other is in ,karnes and ,live ,oak counties in the ,gulf ,coastal ,plain4 ,one of the light-colored uranium minerals1 carnotite1 is a potassium-uranium vanadate1 which has a bright canary-yellow or lemon-yellow color4 ,this mineral is transparent to translucent and has an earthy or a pearly luster4 ,carnotite usually is found as crusts and as powdery masses4 ,it is quite soft and can be scratched with a fingernail4 ,carnotite1 along with tyuyamunite1 autunite1 and several other soft1 yellowish or greenish uranium minerals1 is found in the ,texas ,gulf ,coastal ,plain4 ,these minerals occur in the ,jackson1 ,catahoula1 and ,oakville strata 7which are ,tertiary in age7 in an area extending from ,gonzales ,county to the ,rio ,grande 7in parts of the area indicated by no4 #b and no4 #c on the geologic map1 pp4 #d-e74 ,the largest deposits in this district have been found in the ,karnes ,county area4 ,the ,gulf ,coastal ,plain uranium minerals occur mostly with sandstones and clays in a sequence of strata that contains volcanic ash4 ,it is believed that small scattered amounts of uranium compounds that were present in the volcanic ash sediments were dissolved by seeping underground water4 ,these waters then moved into the sandstones and clays where they deposited the uranium as carnotite and as other uranium minerals4 ,another uranium mineral1 uranophane 7calcium-uranium silicate71 also occurs in ,texas4 ,uranophane has a yellow to yellow-orange color and a pearly to greasy luster4 ,when rubbed across a streak plate1 it leaves a light yellow to a light yellow-orange streak4 ,it is soft enough to be scratched by a copper penny4 ,uranophane has been found in extrusive igneous rocks in northwestern ,presidio ,county in west ,texas4 ,a dark-colored uranium mineral1 pitchblende1 is a variety of the mineral uraninite1 uranium dioxide4 ,pitchblende does not occur with a crystal shape but rather as rounded and irregular-shaped masses4 ,it is brownish black1 greenish black1 or black4 ,if you rub it across a streak plate1 pitchblende leaves a brownish-black streak4 ,this mineral is heavy 7it has a specific gravity of #f4e to #h4e7 and hard 7a pocket knife will not scratch it1 although a steel file will74 ,pitchblende has a submetallic luster and looks dull1 greasy1 or like pitch or tar4 ,small amounts of pitchblende have been found at several places in ,texas4 ,one of these localities is a few miles west of ,burnet in ,burnet ,county in central ,texas4 ,here1 the pitchblende occurs in ,precambrian igneous rocks that are associated with gneiss4 ,in south ,texas1 some fine1 scattered particles of pitchblende have been found about #cbe feet below the surface in ,tertiary 7,pliocene7 sediments that cover the ,palangana salt dome in ,duval ,county4 ,no pitchblende is mined in ,texas4 ,uranophane4 ,see ,uranium ,minerals4 ,vitrophyre4 ,see ,obsidian and ,vitrophyre4 ,volcanic ,ash 7,pumicite7 ,volcanic ash deposits1 which also are known as pumicite1 are loose and powdery4 ,they are made up mostly of material that is thrown into the air when volcanoes erupt4 ,if a volcano erupts with a violent explosion1 the nearby rocks are blown into powder4 ,molten lava also is hurled into the air1 where some of it immediately cools to become tiny bubbles and particles of glass4 ,the winds may carry some of this fine material far away before depositing it4 ,deposits of volcanic ash are white1 bluish1 greenish1 yellowish1 or grayish1 and some of them glisten like snow in the sunlight4 ,they feel rough and gritty4 ,when examined under a microscope1 this material shows the tiny curved and sharp-cornered particles of the broken volcanic glass4 ,deposits of volcanic ash may also contain clay1 silt1 sand1 or other impurities4 ,volcanoes1 which may have been located in the ,davis ,mountains and in other areas of west ,texas and in northern ,mexico1 erupted during ,tertiary time4 ,the volcanic ash that we find at the surface today in some of the ,tertiary formations in ,texas could have come from these volcanoes4 ,tertiary volcanic ash deposits occur in the ,texas ,gulf ,coastal ,plain 7such as in ,brazos1 ,fayette1 ,karnes1 ,polk1 ,starr1 ,trinity1 and other counties7 and in the ,trans-,pecos country of west ,texas4 ,volcanic ash deposits of ,quaternary 7,pleistocene7 age1 which are less than a million years old1 are found in a number of counties on the ,texas ,high ,plains4 ,farther to the east1 ash deposits occur in ,baylor1 ,dickens1 ,kent1 and ,wilbarger counties4 ,this volcanic ash may have come from a volcano that erupted in northern ,new ,mexico during ,quaternary time4 ,volcanic ash or pumicite has several commercial uses4 ,some is used to make pozzolan cement1 and some is used in sweeping compounds1 cleansing and scouring powders1 and abrasive soaps4 ,pumicite has been mined in ,dickens1 ,scurry1 ,starr1 and several other counties of ,texas4 ,wad4 ,see ,manganese ,minerals4 ,wood ,opal4 ,see ,opal4 ,,composition1 ,,hardness1 ,,and ,,specific ,,gravity ,,of ,,some ,,texas ,,minerals ,for convenient reference1 the ,texas minerals described in this book are listed below1 together with their chemical compositions1 specific gravities1 and hardness4 ,you will be able to find similar information about additional minerals in mineralogy textbooks such as those noted on page #bd4 ,mineral ,composition ,specific ,gravity ,hardness ,albite ,na,al,si,o #b4fb #f ,almandite ,fe,al 7,si,o7 #d4b #g ,amphibole ,ca,mg,si,o7,,oh7 #c4j-c4c #a-b asbestos ,anhydrite ,ca,s,o #b4i #c-c ,argentite ,ag,s #g4c #b-b ,azurite ,cu7,,co77,,oh7 #c4gg #c-d ,barite ,ba,s,o #d4e #c-c ,biotite ,k7,mg1 #b4h-c4b #b-c ,fe7,al,si,o7,,oh7 ,braunite #c,mn,mn,o,mn,si,o #d4ge-d4hb #f-f ,calcite ,ca,c,o #b4gb #c ,carnotite ,k,o#b,,uo,v,on,h,o #e4jc #b ,cassiterite ,sn,o #f4h-g4a #f-g ,celestite ,sr,s,o #c4ie-c4ig #c-c ,cerargyrite ,ag,cl #e4e #a-a ,chalcocite ,cu,s #e4e-e4h #b-c ,chalcopyrite ,cu,fe,s #d4a-d4c #c-d ,cinnabar ,hg,s #h4aj #b ,dolomite ,ca,mg7,,co7 #b4he #c-d ,feldspar 7see ,albite1 ,microcline1 ,orthoclase7 ,fluorite ,ca,f #c4ah #d ,galena ,pb,s #g4d-g4f #b ,garnet 7see ,almandite1 ,grossularite7 ,gold ,au #ae4j-ai4c #b-c ,graphite ,c #b4b #a-b ,grossularite ,ca,al7,si,o7 #c4ec #f ,gypsum ,ca,s,o#b;,h,o #b4cb #b ,halite ,na,cl #b4af #b ,hematite ,fe,o #e4bf #a-f ,hollandite ,mn,ba,mn,o #d4g-e #d-f ,limonite ,fe,o7,,oh7n,h,o #c4f-d4j #a-e ,magnetite ,fe,o #e4ah #f ,malachite ,cu,,co7,,oh7 #c4i-d4jc #c-d ,mica 7see ,muscovite1 ,biotite7 ,microcline ,k,al,si,o #b4ed-b4eg #f ,muscovite ,k,al,si,o7,,oh7 #b4gf-c4a #b-b ,opal ,si,on,h,o #a4i-b4b #e-f ,orthoclase ,k,al,si,o #b4eg #f ,pitchblende ,,uo #f4e-h4e #e ,pyrite ,fe,s #e4jb #f-f ,pyrolusite ,mn,o #d4ge #a-b ,quartz ,si,o #b4fe #g ,serpentine ,mg,si,o7,,oh7 #b4dh #c-d ,silver ,ag #aj4e #b-c ,sulfur ,s #b4je-b4ji #a-b ,talc ,mg,si,o7,,oh7 #b4g-b4h #a ,topaz ,al,si,o7,f1,,oh7 #c4d-c4f #h ,tourmaline ,complex silicate of #c4j-c4be #g-g boron and aluminum ,uranophane ,ca,o#b,,uo#b,si,o#g;,h,o #c4h-c4i #b-c ,,books ,,about ,,rocks ,,and ,,minerals ,many books have been written about rocks and minerals4 ,some are listed below1 and it is likely that your librarian will be able to suggest others4 ,nontechnical ,books for ,beginners ,getting ,acquainted ,with ,minerals1 by ,george ,l4 ,english and ,david ,e4 ,jensen4 ,mc,graw-,hill ,book ,company1 ,inc41 ,new ,york1 ,n4 ,y4 7second edition1 #aieh74 ,the ,rock ,book1 by ,carroll ,l4 ,fenton and ,mildred ,a4 ,fenton4 ,doubleday ,company1 ,inc41 ,garden ,city1 ,n4 ,y4 7#aidj74 ,mineral ,collector's ,guide1 by ,david ,e4 ,jensen4 ,ward's ,natural ,science ,establishment1 ,inc41 ,rochester1 ,n4 ,y4 7#aiec74 ,my ,hobby is ,collecting ,rocks and ,minerals1 by ,david ,e4 ,jensen4 ,hart ,book ,company1 ,new ,york1 ,n4 ,y4 7#aiee74 ,rocks and ,minerals1 by ,richard ,m4 ,pearl4 ,barnes ,noble1 ,new ,york1 ,n4 ,y4 7#aief74 #ajja ,questions ,answered ,about the ,mineral ,kingdom1 by ,richard ,m4 ,pearl4 ,dodd1 ,mead ,company1 ,new ,york1 ,n4 ,y4 7#aiei74 ,rocks and ,minerals1 by ,herbert ,s4 ,zim and ,paul ,r4 ,schaffer4 ,simon and ,schuster1 ,inc41 ,new ,york1 ,n4,y4 7#aieg74 ,textbooks and ,other ,reference ,books ,economic ,mineral ,deposits1 by ,alan ,m4 ,bateman4 ,john ,wiley ,sons1 ,inc41 ,new ,york1 ,n4 ,y4 7second edition1 #aiej74 ,a ,textbook of ,mineralogy1 by ,edward ,s4 ,dana1 revised by ,william ,e4 ,ford4 ,john ,wiley ,sons1 ,inc41 ,new ,york1 ,n4 ,y4 7fourth edition1 #aicb74 ,industrial ,minerals and ,rocks 7,nonmetallics ,other ,than ,fuels71 ,joseph ,l4 ,gillson1 ,editor-in-,chief4 ,the ,american ,institute of ,mining1 ,metallurgical1 and ,petroleum ,engineers1 ,new ,york1 ,n4 ,y4 7third edition1 #aifj74 ,dana's ,manual of ,mineralogy1 revised by ,cornelius ,s4 ,hurlbut1 ,jr4 ,john ,wiley ,sons1 ,inc41 ,new ,york1 ,n4 ,y4 7seventeenth edition1 #aiei74 ,mineralogy1 by ,edward ,h4 ,kraus1 ,walter ,f4 ,hunt1 and ,lewis ,s4 ,ramsdell4 ,mc,graw-,hill ,book ,company1 ,inc41 ,new ,york1 ,n4 ,y4 7fifth edition1 #aiei74 ,nonmetallic ,minerals1 by ,raymond ,b4 ,ladoo and ,w4 ,m4 ,meyers4 ,mc,graw-,hill ,book ,company1 ,inc41 ,new ,york1 ,n4 ,y4 7second edition1 #aiea74 ,rocks and ,rock ,minerals1 by ,louis ,v4 ,pirsson1 revised by ,adolph ,knopf4 ,john ,wiley and ,sons1 ,inc41 ,new ,york1 ,n4 ,y4 7third edition1 #aidg74 ,a ,field ,guide to ,rocks and ,minerals1 by ,frederick ,h4 ,pough4 ,houghton ,mifflin ,company1 ,boston1 ,mass4 7third edition1 #aifj74 ,mineral ,facts and ,problems1 by the ,staff of the ,bureau of ,mines4 ,u4 ,s4 ,bureau of ,mines ,bulletin #ehe4 ,u4 ,s4 ,government ,printing ,office1 ,washington1 ,d4 ,c4 7#aifj74 ,selected ,references on ,texas ,rocks and ,minerals ,entries marked with an asterisk are published by the ,bureau of ,economic ,geology1 ,the ,university of ,texas1 ,austin4 ,those not out of print are distributed at nominal sale price1 and a list of publications will be sent on request4 ,these publications can be consulted at many public libraries and ,chamber of ,commerce offices4 99,report on the ,pavitte ,silver-,copper ,prospect in ,burnet ,county1 ,texas1 by ,v4 ,e4 ,barnes4 ,univ4 ,texas1 ,bureau ,econ4 ,geol4 ,mineral ,resource ,survey ,circ4 #e 7#aicf74 99,report on the ,sheridan ,copper ,prospect in ,burnet ,county1 ,texas1 by ,v4 ,e4 ,barnes4 ,univ4 ,texas1 ,bur4 ,econ4 ,geol4 ,mineral ,resource ,survey ,circ4 #i 7#aicf74 99,building ,stones of ,central ,texas1 by ,v4 ,e4 ,barnes1 ,r4 ,f4 ,dawson1 and ,g4 ,a4 ,parkinson4 ,univ4 ,texas ,pub4 #dbdf 7#aidg74 99,iron ,ore in the ,llano ,region1 ,central ,texas1 by ,v4 ,e4 ,barnes4 ,univ4 ,texas1 ,bur4 ,econ4 ,geol4 ,rept4 ,inves4 ,no4 #e 7#aidi74 99,utilization of ,texas ,serpentine1 by ,v4 ,e4 ,barnes1 ,d4 ,a4 ,shock1 and ,w4 ,a4 ,cunningham4 ,univ4 ,texas ,pub4 #ejbj 7#aiej74 99,lead ,deposits in the ,upper ,cambrian of ,central ,texas1 by ,v4 ,e4 ,barnes4 ,univ4 ,texas1 ,bureau ,econ4 ,geol4 ,rept4 ,inves4 ,no4 #bf 7#aief74 99,mineral ,resources of the ,colorado ,river ,industrial ,development ,association ,area1 by ,j4 ,w4 ,dietrich and ,j4 ,t4 ,lonsdale4 ,univ4 ,texas1 ,bureau ,econ4 ,geol4 ,rept4 ,inves4 ,no4 #cg 7#aieh74 99,some ,uranium ,occurrences in ,west ,texas1 by ,d4 ,h4 ,eargle4 ,univ4 ,texas1 ,bureau ,econ4 ,geol4 ,rept4 ,inves4 ,no4 #bg 7#aief74 99,a ,preliminary ,report on the ,stratigraphy of the ,uranium-,bearing ,rocks of the ,karnes ,county ,area1 ,south-,central ,texas1 by ,d4 ,h4 ,eargle and ,j4 ,l4 ,snider4 ,univ4 ,texas1 ,bureau ,econ4 ,geol4 ,rept4 ,inves4 ,no4 #cj 7#aieg74 ,the ,brown ,iron ,ores of ,eastern ,texas1 by ,e4 ,b4 ,eckel4 ,u4 ,s4 ,geol4 ,survey ,bull4 #ijb 7#aich74 99,the ,rustler ,springs ,sulphur ,deposits as a ,source of ,fertilizer1 by ,g4 ,l4 ,evans4 ,univ4 ,texas1 ,bureau ,econ4 ,geol4 ,rept4 ,inves4 ,no4 #a 7#aidf74 ,origin of the ,gulf ,coast ,salt-,dome ,sulphur ,deposits1 by ,herbert ,w4 ,feely and ,j4 ,lawrence ,kulp4 ,bull4 ,amer4 ,assoc4 ,petrol4 ,geol41 vol4 #da1 pp4 #ahjb-ahec 7#aieg74 99,pegmatites of the ,van ,horn ,mountains1 ,texas1 by ,p4 ,t4 ,flawn4 ,univ4 ,texas1 ,bureau ,econ4 ,geol4 ,rept4 ,inves4 ,no4 #i 7#aiea74 99,the ,hazel ,copper-,silver ,mine1 ,culberson ,county1 ,texas1 by ,p4 ,t4 ,flawn4 ,univ4 ,texas1 ,bureau ,econ4 ,geol4 ,rept4 ,inves4 ,no4 #af 7#aieb74 99,basement ,rocks of ,texas and ,southeast ,new ,mexico1 by ,p4 ,t4 ,flawn4 ,univ4 ,texas ,pub4 #efje 7#aief74 99,texas ,miners ,boost ,talc ,output1 by ,p4 ,t4 ,flawn4 ,univ4 ,texas1 ,bureau ,econ4 ,geol4 ,rept4 ,inves4 ,no4 #ce 7#aieh74 99,geology and ,mineral ,deposits of ,pre-,cambrian ,rocks of the ,van ,horn ,area1 ,texas1 by ,p4 ,b4 ,king and ,p4 ,t4 ,flawn4 ,univ4 ,texas ,pub4 #ecja 7#aiec74 99,igneous ,rocks of the ,balcones ,fault ,region of ,texas1 by ,j4 ,t4 ,lonsdale4 ,univ4 ,texas ,bull4 #bgdd 7#aibg74 ,mineral ,resources of the ,llano-,burnet ,region1 ,texas1 with an ,account of the ,pre-,cambrian ,geology1 by ,sidney ,paige4 ,u4 ,s4 ,geol4 ,survey ,bull4 #dej 7#aiaa74 99,mineral ,resources of the ,texas ,coastal ,plain 7,preliminary ,report71 by ,j4 ,m4 ,perkins and ,j4 ,t4 ,lonsdale4 ,univ4 ,texas1 ,bureau ,econ4 ,geol4 ,mineral ,resource ,circ4 #ch 7#aiee74 ,geology and ,ore ,deposits of the ,shafter ,mining ,district1 ,presidio ,county1 ,texas1 by ,c4 ,p4 ,ross4 ,u4 ,s4 ,geol4 ,survey ,bull4 #ibh-;,b 7#aidc74 99,the ,geology of ,texas1 ,vol4 ,,ii1 ,structural and ,economic ,geology1 by ,e4 ,h4 ,sellards1 ,c4 ,l4 ,baker1 and others4 ,univ4 ,texas ,bull4 #cdja 7#aice74 99,texas ,mineral ,resources1 by ,e4 ,h4 ,sellards and others4 ,univ4 ,texas ,pub4 #dcja 7#aidf74 99,geological ,resources of the ,trinity ,river ,tributary ,area in ,texas and ,oklahoma1 by ,h4 ,b4 ,stenzel1 ,a4 ,e4 ,weissenborn1 and others4 ,univ4 ,texas ,pub4 #dhbd 7#aidh74 ,uranium at ,palangana ,salt ,dome1 ,duval ,county1 ,texas1 by ,a4 ,d4 ,weeks and ,d4 ,h4 ,eargle4 ,in ,u4 ,s4 ,geol4 ,survey ,prof4 ,paper #djj-;,b 7#aifj74 ,geology of the ,quicksilver ,deposits of the ,terlingua ,district1 ,texas1 by ,r4 ,g4 ,yates and ,g4 ,a4 ,thompson4 ,u4 ,s4 ,geol4 ,survey ,prof4 ,paper #cab 7#aiei74 ,,glossary ,amorphous--without crystalline structure and therefore without regular form4 ,balcones fault zone--a system of faults extending from north of ,waco in ,mc,lennan ,county1 through ,travis and ,bexar counties1 to near ,del ,rio in ,val ,verde ,county 7see p4 #db74 ,boulder--a large rock or mineral fragment that has a diameter greater than #bef millimeters 7about #aj inches74 ,breccia--a rock made up of sharp-cornered1 cemented fragments with diameters greater than #b millimeters 7about _/ of an inch74 ,cambrian--the earliest period of the ,paleozoic ,era 7see p4 #c74 ,cenozoic--the present era1 one of the great divisions of geologic time 7see p4 #c74 ,this era began about #fc million years ago4 ,clastic--made up of broken fragments of rocks or minerals4 ,cleavage--occurs when minerals split along smooth flat surfaces that are parallel to possible crystal faces4 ,these planes as well as crystal faces are controlled by the crystal lattice or atomic structures of the minerals4 ,cleavage fragment--a mineral specimen that has been broken along its planes of cleavage4 ,cobble--a rock or mineral fragment that has a diameter between #fd and #bef millimeters 7about #b and #aj inches74 ,conchoidal--a curved fracture surface shaped like the inside of a shell or spoon4 ,conglomerate--a rock composed of cemented1 rounded rock or mineral fragments1 most of which are of gravel size4 ,cretaceous--the third and latest period of the ,mesozoic ,era 7see p4 #c74 ,cryptocrystalline--made up of tiny crystalline particles that are too small to be distinguished even under high magnification4 ,crystalline--having a definite1 orderly internal structure4 ,cube--a solid that has six equal1 square sides4 ,dodecahedron--a solid that has twelve plane1 four-sided faces4 ,element--a basic building block of all matter1 which cannot be separated into different substances by ordinary chemical means4 ,eocene--the second epoch of the ,tertiary ,period 7see p4 #c74 ,epoch--a unit of geologic time that is a subdivision of a period4 ,era--a major division of geologic time1 which consists of several periods4 ,extrusive rocks--igneous rocks formed from magma that was extruded on the earth's surface4 ,fault--a break in the rocks or strata of the earth's crust along which movement or slippage has taken place4 ,fluid--a substance made up of particles that can move freely about2 it can be a liquid or a gas4 ,formation--rocks or strata that are recognized and mapped as a unit4 ,fracture--the kind of surface obtained if a mineral is broken in a different direction from that of the cleavage or parting4 ,commonly1 fracture surfaces are rough1 uneven1 or curved1 whereas cleavage surfaces are smooth4 ,geologic map 7areal7--shows the extent and distribution of formations exposed at the earth's surface4 ,granular--the texture of a rock or mineral that is made up of visible grains4 ,if all the grains are about the same size1 the term equigranular is used4 ,granule--a rock or mineral fragment that has a diameter of from #b to #d millimeters 7about _/ to _/ of an inch74 ,gravel--uncemented rock or mineral fragments that have diameters greater than #b millimeters 7about _/ of an inch74 ,gulf ,coastal ,plain--an area that extends1 in ,texas1 from the ,gulf of ,mexico to the ,balcones fault zone and in which ,quaternary1 ,tertiary1 and ,upper ,cretaceous strata crop out at the surface 7see p4 #db74 ,high ,plains--an area in northwest ,texas extending from the ,pecos ,river valley north to the ,oklahoma-,texas boundary 7see p4 #db74 ,igneous rocks--rocks formed by the cooling and hardening of hot1 molten rock material4 ,intrusive rocks--igneous rocks that have formed below the surface of the earth4 ,lava--molten rock material that has poured out onto the earth's surface from volcanoes2 also the rock that is formed after the molten material has cooled and hardened4 ,llano uplift--an area in central ,texas where ,precambrian and early ,paleozoic rocks occur at the earth's surface 7see p4 #db74 ,magma--hot1 molten rock material from which igneous rocks form4 ,massive--in a mass1 without a regular or complete form4 ,mesozoic--an era1 one of the great divisions of geologic time 7see p4 #c74 ,this era began about #bcj million years ago and lasted until about #fc million years ago4 ,metamorphic rock--rock formed from igneous or sedimentary rocks that are altered by heat1 pressure1 and fluids below the earth's surface4 ,miocene--the fourth epoch of the ,tertiary ,period 7see p4 #c74 ,mississippian--the fifth period of the ,paleozoic ,era 7see p4 #c74 ,nodule--a small1 rounded mass or lump4 ,octahedron--a solid that has eight triangular faces4 ,opaque--no light can pass through4 ,ordovician--the second period of the ,paleozoic ,era 7see p4 #c74 ,paleozoic--an era1 one of the great divisions of geologic time 7see p4 #c74 ,this era began at the end of ,precambrian time and lasted until about #bcj million years ago4 ,parting--occurs when a mineral breaks along a flat surface that is not a true cleavage plane4 ,pebble--a rock or mineral fragment that has a diameter between #d and #fd millimeters 7about _/ and #b inches74 ,pennsylvanian--the sixth period of the ,paleozoic ,era 7see p4 #c74 ,period--a unit of geologic time1 a subdivision of an era4 ,permian--the last period of the ,paleozoic ,era 7see p4 #c74 ,physiographic outline map--shows location of natural regions 7p4 #db74 ,playa lake--a temporary shallow lake in a nearly level1 closed basin1 which has no drainage outlet4 ,pleistocene--the first epoch of the ,quaternary ,period 7see p4 #c74 ,pliocene--the last epoch of the ,tertiary ,period 7see p4 #c74 ,precambrian--comprises the ,early and the ,late ,precambrian ,eras1 the earliest great divisions of geologic time4 ,rocks that formed more than #fjj million years ago are known as ,precambrian rocks4 ,pyritohedron--a solid that has twelve #e-sided faces4 ,quaternary--the present period of geologic time2 the second period of the ,cenozoic ,era 7see p4 #c74 ,recent--the present epoch of geologic time2 the second epoch of the ,quaternary ,period 7see p4 #c74 ,sectile--describes material1 such as soap1 that can be cut smoothly with a knife4 ,sediments--material deposited by water1 wind1 or ice on the earth's surface4 ,sedimentary rocks--rocks made up of sediments4 ,series--a subdivision of a system that includes all rocks formed during an epoch4 ,specific gravity--the ratio of the weight of a substance to the weight of an equal volume of water4 ,streak--the color of the powder of a mineral4 ,system--all rocks formed during a period4 ,tertiary--the first period of the ,cenozoic ,era 7see p4 #c74 ,translucent--light will pass through1 but objects cannot be seen4 ,transparent--light will pass through1 and objects can be seen4 ,trans-,pecos--area of ,texas located west of the ,pecos ,river 7see p4 #db74 ,volcanic rocks--igneous rocks that have formed on the earth's surface2 extrusive rocks4 ,index ,a acid3 #ah1 #bb acid tests 7see also chemical tests73 #ah actinolite3 #hb schist3 #hg agate3 #ba1 #hc agatized wood3 #bj1 #hd age1 earth's crust3 #b alabaster3 #fe albite3 #ee alkali lakes3 #ff1 #hc ,allamoore3 #hg1 #ic alloys3 #eb almandite3 #eh ,alpine3 #eg1 #gh1 #hb1 #ij ,altuda ,mountain3 #eg1 #ij amalgam3 #ea amazonstone3 #ef amethyst3 #cg1 #hb ,amethyst ,hill3 #hb amorphous minerals1 definition of3 #ae amphibole asbestos3 #bi1 #cc1 #cd1 #dd ,anderson ,county3 #gj ,andrews ,county3 #hc ,angelina ,county3 #eb anhydrite3 #bh1 #bi1 #cj1 #ce1 #cf1 #dc1 #fe1 #ff1 #ia anthophyllite3 #ic antigorite3 #hg ,apache ,mountains3 #fe apatite3 #gb ,archer ,county3 #eb argentite3 #bf1 #bg1 #hi1 #ij argentiferous galena3 #eg1 #ij ,armstrong ,county3 #gi asbestos3 #dc amphibole3 #bi1 #cc1 #cd1 #dd chrysotile3 #dd1 #hg ash1 volcanic3 #da1 #ig augite3 #de ,austin3 #df1 #fb autunite3 #ig azurite3 #ce1 #eb1 #ec1 #ed ,b ,babyhead3 #gb ,balcones fault zone3 #aj1 #db1 #de1 #hh ,bandera ,county3 #de ,baringer ,hill pegmatite3 #hj barite3 #ae1 #bh1 #ca1 #cb1 #ce1 #cf1 #ch1 #dd1 #dh barium3 #de ,barrilla ,mountains3 #he ,baylor ,county3 #de1 #eb1 #ih basal cleavage3 #gf1 #ic1 #id basalt3 #dj1 #da1 #de basement3 #b1 #i ,bastrop ,county3 #fd ,bell ,county3 #fi bentonite3 #ea1 #eb beryllium3 #hj ,big ,bend area3 #i1 #db1 #ea1 #ge1 #gg ,big ,branch ,gneiss3 #ei biotite3 #cb1 #cc1 #cd1 #gf ,bird mine3 #ij ,blanco ,county3 #dd1 #eh1 #ei1 #fa1 #gj1 #hc1 #hg1 #id black mica 7see biotite7 blast furnace3 #gj1 #ga blast sand3 #hf bleaching clay3 #ea blue copper3 #ec ,boerne3 #ah ,boling salt dome3 #ia books about rocks and minerals3 #bd1 #ci1 #ajj brass3 #eb braunite3 #bf1 #bg1 #gc ,brazoria ,county3 #ia ,brazos ,county3 #ih ,brazos ,river1 ,double ,mountain ,fork and ,salt ,fork of3 #dc breccia3 #ab1 #da1 #ha ,brewster ,county3 #de1 #df1 #dg1 #di1 #ej1 #eg1 #fe1 #gj1 #gf1 #gg1 #gh1 #hb1 #hd1 #he1 #ij brimstone3 #ij bronze3 #eb ,brooks ,county3 #fd ,brown1 ,thomas ,e43 vii ,brown ,county3 #de1 #di brown iron ore 7see also limonite73 #gj ,bryan ,mound salt dome3 #ia building sand3 #he ,burnet3 #ef1 #hj1 #ig ,burnet ,county3 #ah1 #ec1 #ed1 #ee1 #ef1 #eh1 #ei1 #fj1 #fa1 #fb1 #fc1 #gj1 #gf1 #gi1 #hj1 #ha1 #hb1 #hc1 #ig ,c calcareous sinter and tufa3 #df calcite3 #g1 #ad1 #ah1 #ai1 #bj1 #ba1 #bh1 #cj1 #ca1 #cb1 #cd1 #ce1 #cf1 #ch1 #de1 #df1 #di1 #ej1 #ea1 #ed1 #ef1 #fd1 #fh1 #ge1 #hf1 #hg1 #hh1 #ia dog-tooth spar3 #df1 #dg ,caldwell ,county3 #hh caliche3 #df1 #dg ,cambrian3 #b1 #c1 #ee1 #eh1 #fi1 #hf ,cameron ,county3 #ff ,capitan reef limestone3 #a ,capitol building1 ,texas ,state3 #fb cap-rock3 #ia1 #ib carats3 #fj carbon3 #fb1 #fc carbonized wood3 #bj carnotite3 #cj1 #ie1 #ig ,carrizo ,mountains3 #fg1 #hc1 #ie ,casey1 ,miss ,josephine3 vii ,cass ,county3 #gj cassiterite3 #bg1 #cb1 #dg ,castile ,gypsum3 #ia ,catahoula strata3 #gh1 #hf1 #ig cave deposits3 #ah caves3 #ah1 #ai1 #di ,caverns of ,sonora3 #ai ,cedar ,lake3 #hc ,cedar ,park3 #gj celestite3 #bj1 #bh1 #cj1 #ce1 #ch1 #dh cementing materials3 #ab1 #df1 #fg1 #gj1 #hf cerargyrite3 #bh1 #bi1 #cd1 #cg1 #hi1 #ij cerium3 #hj chalcedony 7see also agate73 #af1 #ba1 #de1 #gh1 #ha1 #hc1 #hf chalcocite3 #ba1 #bf1 #bg1 #eb1 #ed1 #gc1 #hi chalcopyrite3 #bf1 #eb1 #ec1 #ed1 #fj chalk3 #dj1 #fi chemical elements3 #g chemical sediments3 #ab chemical tests3 #ah1 #dc1 #df1 #dg1 #di1 #eb1 #ec1 #ed1 #fd1 #fh1 #ge ,cherokee ,county3 #gj chert3 #ai1 #dj1 #ha1 #hd chessylite3 #ec china clay3 #ea ,chinati ,mountains3 #de1 #eg1 #fb1 #he ,chisos ,mountains3 #de1 #fb1 #he chlorite3 #ic schist3 #hg chrysotile3 #dd1 #hg1 #hh asbestos3 #dd1 #hg cinnabar3 #cf1 #di clastic rocks3 #ab clay3 #dj1 #de1 #di1 #ea1 #gj1 #hf1 #hh1 #ia bentonite3 #de1 #ea1 #eb bleaching clay3 #ea china clay3 #ea kaolin3 #ea clay minerals3 #ea ,clay ,county3 #eb ,clear ,creek area3 #fc cleavage3 #ag basal3 #gf1 #ic1 #id cubic3 #ag1 #eg1 #fe octahedral3 #ef1 #eg pyramidal3 #gd rhombohedral3 #df1 #ed ,clemens salt dome3 #ia ,coal ,creek serpentine3 #hg1 #hh ,coke ,county3 #bj1 #di ,coleman ,county3 #hf color of minerals3 #af columns3 #ah ,comal ,county2 #de1 #fi ,comanche ,county3 #di common opal3 #gh composition of minerals3 #ii concretions3 #ai conglomerate3 #aa1 #ab1 #da1 #ee1 #ha gneiss3 #ei contact metamorphism3 #ac copper3 #eb1 #ei1 #ij blue copper3 #ec glance3 #eb green copper carbonate3 #ec minerals3 #eb pyrites3 #ec yellow copper ore3 #ec coquina3 #da1 #fi ,crane ,county3 #ff ,cretaceous3 #b1 #c1 #bj1 #ba1 #de1 #df1 #di1 #ea1 #ee1 #eg1 #fj1 #fe1 #fh1 #fi1 #gj1 #gd1 #ge1 #hd1 #hf1 #hh cryptocrystalline quartz3 #ha1 #hd crystal1 definition of3 #ad crystals1 twinned3 #df1 #fc crystalline minerals1 definition of3 #ad ,crosby ,county3 #gi crust1 earth's3 #b cubic cleavage3 #ag1 #eg1 #fe ,culberson ,county3 #a1 #de1 #eb1 #ec1 #eh1 #ei1 #fj1 #fe1 #ge1 #gi1 #hc1 #ij1 #ia1 #ie ,d ,daingerfield3 #gj ,dallas3 #hh ,damon ,mound salt dome3 #ia daughter elements3 #ie1 #if ,davis ,mountains3 #de1 #he1 #ih ,delaware ,mountains3 #fe diamond3 #fb diatomaceous earth3 #gh diatomite3 #dj1 #gh ,dickens ,county3 #gi1 #ih ,dietrich1 ,john ,w43 vii dikes3 #gb1 #gi dog-tooth spar3 #df1 #dg dolomite3 #bi1 #cj1 #cb1 #cf1 #ch1 #dj1 #dc1 #ed1 #fh1 #ge1 #hg1 #hh dolomitic limestone3 #fh ,double ,mountain ,fork of the ,brazos ,river3 #dc double refraction3 #df dravite3 #ie ,duval ,county3 #gh1 #hc1 #ia1 #ig dynamic metamorphism3 #ad ,e ,eagle ,flat3 #ic ,eagle ,mountains3 #ef1 #eg1 #ij1 #ie earth's crust3 #b east ,texas3 #gj ,ector ,county3 #gi ,edwards ,limestone3 #fh1 #fi1 #hd effervescence in acid3 #ah ,el ,capitan ,peak3 #a elements3 #g1 #h1 #ie1 #if ,ellenburger strata3 #ee1 #hd ,el ,paso3 #dh1 #eh1 #he1 #hh ,el ,paso ,county3 #dh1 #eh1 #fb epoch3 #b era3 #b erosion3 #aa extrusive igneous rocks1 definition of3 #i ,f ,fairland3 #ed ,falfurrias3 #fd ,fannett salt dome3 #ia ,fayette ,county3 #eb1 #fd1 #hd1 #ih feldspar3 #h1 #bi1 #cj1 #ca1 #cc1 #cd1 #ce1 #cg1 #de1 #ea1 #ee1 #ei1 #fa1 #gb1 #gi1 #hj1 #he1 #hf albite3 #ee amazonstone3 #ef microline3 #ee1 #ef1 #fa orthoclase3 #ee1 #fa fibrous gypsum3 #fd1 #fe filtering sand3 #hf ,fisher ,county3 #bj1 #di1 #eh1 #fe ,flawn1 ,peter ,t43 vii flint3 #hd flowstone3 #ah flow structure3 #he fluorite3 #bi1 #cb1 #cd1 #ce1 #cg1 #ch1 #ef1 #gb ,foard ,county3 #eb1 #eh foliation3 #hg fool's gold3 #ec1 #hj formation3 #f ,fort ,bend ,county3 #ia ,fort ,worth3 #hh fracture of minerals3 #ag fragmental rocks3 #ab ,franklin ,mountains3 #dh1 #eh1 #fb1 #he ,frasch method of sulfur mining3 #ia1 #ib ,fredonia3 #fg ,freer3 #gh1 #hc furnace 7see blast furnace2 open-hearth furnace7 ,g gabbro3 #hg gneiss3 #ei ,gaines ,county3 #hc galena3 #ag1 #bf1 #eg1 #hi1 #ij argentiferous3 #eg1 #ij ,galveston ,county3 #ia ,gamble prospect3 #fg gamma rays3 #if garnet3 #ca1 #cc1 #cd1 #cg1 #eh1 #he1 #hg ,garza ,county3 #ig ,geiger counter3 #if gemstones3 #ef1 #eh1 #gg1 #hb1 #id1 #ie geode3 #ai1 #df1 #di1 #hb geologic map3 #d1 #e geologic time scale3 #b1 #c geologists3 #b geology3 #b ,gillespie ,county3 #dd1 #de1 #ee1 #eh1 #ei1 #fj1 #fa1 #fe1 #gf1 #hj1 #hb1 #hc1 #hg1 #hh1 #id glass1 natural3 #i1 #gg1 #he1 #ig glass sand3 #he-hf glauconite3 #gj1 #gc glossary3 #ajb-ajc gneiss3 #ac1 #da1 #ee1 #ef1 #ei1 #fg1 #gb1 #gc1 #gd1 #gf1 #ha1 #hg1 #ic1 #ig conglomerate3 #ei gabbro3 #ei granite3 #ei hornblende3 #ei mica3 #ei goethite3 #gj gold3 #bf1 #ea1 #ei1 #hj1 #ij fool's3 #ec1 #hj ,gonzales ,county3 #eb1 #hd1 #ig ,grand ,saline salt dome3 #ff granite3 #b1 #h1 #i1 #ac1 #da1 #dh1 #ee1 #ef1 #ei1 #fa1 #gb1 #gf1 #gi1 #ha1 #he1 #hg1 #id1 #ie gneiss3 #ei opaline3 #gb ,granite ,mountain3 #fb ,granite ,shoals ,lake3 #ef granules3 #he graphite3 #bf1 #bg1 #fb1 #ge schist3 #fc1 #hg gravel3 #ab1 #da1 #ia green copper carbonate3 #ec greensands3 #gj ,grit3 #id grossularite3 #eh group3 #f ,guadalupe ,mountains1 ,peak3 #a ,gulf ,coast3 #ia ,gulf ,coastal ,plain3 #bj1 #db1 #dc1 #df1 #ea1 #eb1 #eh1 #fj1 #fe1 #ff1 #gh1 #hc1 #hd1 #hf1 #hg1 #ib1 #ig1 #ih ,gulf salt dome3 #ia ,gyp ,hill salt dome3 #fd gypsite3 #fe gypsum3 #bh1 #bi1 #cj1 #cb1 #cf1 #cg1 #dc1 #fc1 #gj1 #he1 #ia alabaster3 #fe fibrous gypsum3 #fd1 #fe rock gypsum3 #dj1 #fe rosettes3 #fc1 #fd satin spar3 #fd selenite3 #fc1 #fd1 #gf ,h halite3 #bh1 #cb1 #cd1 #ce1 #cf1 #ch1 #dj1 #dc1 #fe1 #ia hand lens3 #bb ,hardeman ,county3 #eb1 #eh1 #fd1 #fe hardness of minerals3 #af1 #ii ,harris1 ,bill ,m43 vii ,harris1 ,john ,s43 vii ,harris ,county3 #fe1 #ff ,hartley ,county3 #gi ,hays ,county3 #de ,hazel ,formation3 #eb ,hazel mine3 #eb1 #ec1 #ij ,heath mine3 #fj hematite3 #bf1 #bg1 #cf1 #ff1 #ge1 #hd1 #hf specular3 #fg ,henderson ,county3 #gj ,high ,island salt dome3 #ia ,high ,plains3 #db1 #df1 #ea1 #fe1 #ff1 #gh1 #gi1 #hc1 #hg1 #ig1 #ih ,hockley salt dome3 #fe1 #ff hollandite3 #bf1 #bg1 #bh1 #gc1 #gd ,honey ,creek area3 #hg hornblende3 #h1 #ei1 #fa gneiss3 #ei schist3 #hg horn silver3 #ij ,hoskins ,mound salt dome3 #ia ,houston3 #gj ,howard ,county3 #de1 #fj ,hudspeth ,county3 #de1 #eb1 #ec1 #ee1 #ef1 #eg1 #eh1 #ei1 #fj1 #fb1 #fe1 #ff1 #fg1 #ge1 #gi1 #hb1 #hc1 #ij1 #ic1 #ie ,humphreys1 ,alan3 vii ,hutchinson ,county3 #ff hyalite3 #gh hydrochloric acid 7see also chemical tests73 #ah1 #bb ,i identification charts3 #bd igneous rocks1 definition of3 #i intrusive igneous rocks1 definition of3 #i ,irion ,county3 #fj iron3 #ei1 #ff1 #fg1 #fi1 #gj1 #gb1 #gc ,iron ,mountain3 #gc ,j ,jackson strata3 #ig jasper3 #ha1 #hd ,jasper ,county3 #eb ,jeff ,davis ,county3 #de1 #ea1 #gd1 #gh1 #he ,jefferson ,county3 #ia ,johnson ,county3 #fi ,k kaolin3 #ea kaolinite3 #ea karats3 #fj ,karnes ,county3 #ig1 #ih ,katemcy3 #id ,kendall ,county3 #ah ,kenedy ,county3 #ff ,kent ,county3 #ih kidney ore3 #fg ,king1 ,elbert ,a41 ,jr43 vii ,king ,county3 #eb ,kinney ,county3 #de ,kleberg ,county3 #ff ,knox ,county3 #eb ,l labeling rock and mineral specimens3 #bc labradorite3 #de ,lake ,buchanan3 #hj1 #hc ,lamb ,county3 #gi ,lampasas ,county3 #bj1 #dh1 #di1 #hh lava3 #i lead3 #eg1 #ij ,leakey3 #ea ,lee ,county3 #fd1 #hd lens3 #bb ,liberty ,county3 #ia lime3 #fi limestone3 #ab1 #ac1 #ah1 #ai1 #dj1 #dc1 #de1 #df1 #di1 #ed1 #ee1 #ef1 #eg1 #eh1 #fj1fh1 #gd1 #ge1 #gf1 #hd1 #hh1 #ij1 #ia chalk3 #fi dolomitic3 #fh lithographic3 #fi olitic3 #fi pulverulent3 #da1 #fi limonite3 #ai1 #bg1 #bh1 #cj1 #cb1 #gj1 #ge1 #ha1 #hf lithographic limestone3 #fi ,live ,oak ,county3 #de1 #ig llanite3 #da1 #ga1 #ha ,llano3 #fj1 #fc1 #gb1 #gc1 #ie ,llano ,county3 #dd1 #de1 #ee1 #ef1 #eh1 #ei1 #fj1 #fa1 #fc1 #gj1 #ga1 #gb1 #gc1 #gf1 #gh1 #hj1 #hc1 #hg1 #hh1 #id1 #ie ,llano uplift3 #aj1 #ah1 #db1 #dd1 #eb1 #ee1 #ef1 #eh1 #ei1 #fj1 #fa1 #fb1 #fc1 #fg1 #gj1 #gb1 #gf1 #gg1 #gi1 #hj1 #hb1 #hc1 #hd1 #he1 #hf1 #hg1 #hh1 #ic1 #id1 #ie ,lone ,grove3 #fc ,lone ,star3 #gj ,longhorn ,cavern3 #ah ,long ,point salt dome3 #ia ,lonsdale1 ,john ,t43 vii luster of minerals3 #af ,lytton ,springs oil field3 #hh ,m ,macon1 ,james ,w43 vii magma3 #i magnesium1 source of3 #ee magnetite3 #bh1 #fg1 #gj1 #gb1 #he1 #hg1 #ic malachite3 #cd1 #eb1 #ec1 #ed ,malone ,mountains3 #fe manganese3 #gc1 #ge map-- geologic3 #d-e physiographic outline3 #db ,marathon area3 #db1 #hd1 #hh marble3 #ac1 #ad1 #dj1 #df1 #ed1 #ge ,marble ,falls3 #fb ,marion ,county3 #gj martite3 #fg ,mason3 #gd ,mason ,county3 #dh1 #ee1 #eh1 #ei1 #fj1 #fa1 #fg1 #gb1 #gc1 #gd1 #gf1 #hj1 #hc1 #id masses1 crystalline1 definition of3 #ad ,matagorda ,county3 #ia ,mayfield prospect3 #gd ,mc,culloch ,county3 #hh ,medina ,county3 #de ,menard ,county3 #fe mercury3 #h1 #di1 #ej1 #ea metamorphic rocks1 definition of3 #ab metamorphism1 definition of3 #ac1 #ad mica 7see also biotite2 muscovite73 #h1 #fj1 #fa1 #fc1 #fd1 #gb1 #gf1 #gi1 #hj1 #he1 #hg mica gneiss and_/or schist3 #ei1 #gf1 #hg ,mica ,mine area3 #eh1 #gf1 #gg1 #gi microcline3 #ee1 #ef1 #fa milky quartz3 #hb ,mills ,county3 #di mineral identification charts3 #bd mineralogists3 #b minerals1 definition of3 #g1 #ad1 #ae ,mississippian3 #c1 #ee1 #fi1 #hh ,mitchell ,county3 #ff ,mohs scale of hardness3 #af1 #ag molybdenite3 #fc montmorillonite3 #ea ,morris ,county3 #gj moss agate3 #hd ,moss ,bluff salt dome3 #ia muriatic acid3 #bb muscovite3 #ca1 #cb1 #cd1 #cg1 #ch1 #gf ,n ,nacogdoches ,county3 #gj ,nash salt dome3 #ia native elements3 #h native mercury3 #h native silver3 #bf1 #hi nodule3 #ai1 #dd1 #di ,nolan ,county3 #bj1 #di1 #fc1 #fd1 #fe north-central ,texas3 #dc1 #ea1 #eb1 #fi1 #gj1 #hf1 #hh ,o ,oakville strata3 #ig obsidian3 #dj1 #gg occurrence of minerals3 #ad ocher1 red3 #fg octahedral cleavage3 #ef1 #eg oil shale3 #hh olivine3 #hh onyx3 #hd olitic limestone3 #fi opal3 #ba1 #bi1 #cj1 #ca1 #ce1 #cg1 #ch1 #gh1 #hf opaline granite3 #gb opalized wood3 #bj1 #gh1 #hd open-hearth furnace3 #gj1 #gb ,orchard salt dome3 #ia ,ordovician3 #c1 #ee1 #fi1 #hd orthoclase3 #ee1 #fa ,p ,packsaddle ,mountain3 #hg ,packsaddle ,schist3 #hg ,palangana salt dome3 #ia1 #ig paleontologists3 #b parting of minerals3 #ag ,pavitte prospect3 #ec ,pecos ,county3 #i ,pecos ,river3 #fe1 #gd1 #ge pegmatite3 #da1 #ee1 #ef1 #eh1 #gf1 #gg1 #gi1 #ha1 #id1 #ie ,baringer ,hill3 #hj ,pennsylvanian3 #c1 #de1 #ea1 #ee1 #fi1 #gj1 #hf1 #hh peridotite3 #hh period3 #b ,permian3 #a1 #c1 #bj1 #ba1 #dc1 #de1 #di1 #eb1 #ee1 #fd1 #fe1 #fi1 #hf1 #ij ,permian basin3 #db1 #dc1 #fe1 #ff petrified wood3 #bj1 #eb1 #gh1 #hd petrologists3 #b phenocrysts3 #he physiographic outline map3 #db pitchblende3 #bh1 #ie1 #ig placer deposits3 #fj ,plata ,verde mine3 #ij ,polk ,county3 #ih porphyritic rocks3 #de1 #he portland cement3 #ea1 #fi1 #hh ,precambrian3 #b1 #c1 #dd1 #de1 #dh1 #eb1 #ec1 #ef1 #eh1 #ei1 #fj1 #fa1 #fc1 #fg1 #gb1 #gd1 #ge1 #gf1 #gi1 #hb1 #he1 #hg1 #ic1 #id1 #ie1 #ig precious opal3 #gh ,presidio ,county3 #de1 #di1 #eg1 #fj1 #fb1 #fg1 #gg1 #gh1 #hd1 #he1 #ij1 #ig ,presidio mine3 #fj1 #ij properties of minerals3 #ad pseudomorphs3 #gj1 #hg pulverulent limestone3 #da1 #fi pumicite3 #ig1 #ih pyramidal cleavage3 #gd pyrite3 #ai1 #bf1 #fj1 #gj1 #hj1 #hg pyrolusite3 #bg1 #gc1 #gd pyroxene3 #de ,q quartz3 #h1 #bj1 #bi1 #cj1 #ca1 #cc1 #cf1 #cg1 #ch1 #dh1 #ea1 #ed1 #ef1 #ei1 #fj1 #fa1 #fh1 #gb1 #gh1 #gi1 #hj1ha1 #hd1 #he1 #hf1 #id agate3 #ba1 #hc1 #hd amethyst3 #cg1 #hb chalcedony3 #af1 #ba1 #de1 #gh1 #ha1 #hc1 #hf chert3 #ai1 #dj1 #ha1 #hd cryptocrystalline3 #ha1 #hd jasper3 #ha1 #hd milky3 #hb onyx3 #hd rock crystal3 #hb rose3 #hb1 #hc smoky3 #hb1 #hc quartzite3 #da1 #gd1 #hd1 #hf ,quaternary3 #b1 #c1 #ea1 #gi1 #ih quicksilver 7see also mercury73 #ej ,quitman ,mountains3 #eg1 #eh1 #fj1 #fb1 #fg1 #hb1 #ij ,r radioactivity3 #ie1 #if ,real ,county3 #di1 #ea ,recent3 #b1 #c 8red beds03 #eb red ocher3 #fg reef limestone3 #a1 #ab ,reeves ,county3 #fe1 #he1 #ia reference books3 #bd1 #ci1 #ajj refraction1 double3 #df rhombohedral cleavage3 #df1 #ed rhyolite3 #dj1 #he porphyry3 #da1 #he ,rio ,grande3 #hc1 #hd1 #ig rock-- crystal3 #hb gypsum3 #dj1 #fe identification charts3 #ci salt3 #dj1 #fe units3 #b rocks1 definition of3 #h ,rodda1 ,peter ,u43 vii rose quartz3 #hb1 #hc rosettes3 #fc1 #fd ,rustler ,springs area3 #ia rutile3 #hb ,s salt 7see also halite73 #dj1 #fe1 #ff domes3 #dc1 #eh1 #fd1 #ff1 #ia1 #ig lakes3 #ff rock3 #dj1 #fe ,salt ,fork of the ,brazos ,river3 #dc sand3 #da1 #eh1 #gb1 #he1 #ia blast3 #hf building3 #he filtering3 #hf glass3 #he-hf sandstone3 #da1 #eb1 #ee1 #eh1 #ei1 #gb1 #gf1 #ha1 #hd1 #hf ,sandy ,creek3 #fj ,san ,saba ,county3 #gj1 #hh ,santa ,anna3 #hf satin spar3 #fd ,satorsky1 ,cyril3 vii scale1 geologic time3 #b1 #c scintillation counter3 #if schist3 #da1 #ef1 #eh1 #gb1 #gc1 #gf1 #ha1 #hg1 #ic actinolite3 #hg graphite3 #fc1 #hg hornblende3 #hg mica3 #gf1 #hg talc3 #hg1 #ic schistosity3 #hg schorl3 #ie ,scurry ,county3 #ih sedimentary rocks1 definition of3 #aj sediments3 #h1 #aa chemical3 #ab selenite gypsum3 #fc1 #fd1 #gf series3 #b serpentine3 #cd1 #dj1 #dd1 #ge1 #hg1 #ic serpentinite3 #hg ,seven ,heart ,gap3 #de ,shafter3 #eg1 #eh1 #fj1 #fg1 #ij ,shafter ,lake3 #hc shale3 #ab1 #ac1 #ai1 #dj1 #de1 #gj1 #hg1 #hh oil3 #hh ,shelby1 ,cader ,a43 vii ,sheridan prospect3 #ec siderite3 #gj ,sierra ,blanca3 #eh1 #fg1 #hb silicified wood 7see also petrified wood73 #bj silt3 #ab1 #he siltstones3 #ab silver3 #bf1 #eb1 #eg1 #ei1 #hi glance3 #ij horn3 #ij minerals3 #hi native3 #bf1 #hi ,silver ,creek3 #eh sinter1 calcareous3 #df slate3 #ac ,smith ,county3 #gj smoky quartz3 #hb1 #hc soapstone3 #dj1 #ic soils3 #aj ,solitario uplift3 #db1 #hh ,sonora3 #ah ,caverns of3 #ai specific gravity3 #ah1 #ii specular hematite3 #fg sphalerite3 #eg ,spiller mine3 #gd ,spindletop salt dome3 #ia ,spring ,creek3 #ef stalactites3 #ah1 #ai stalagmites3 #ah1 #ai ,starr ,county3 #hd1 #ih static metamorphism3 #ac ,stonewall ,county3 #eb streak or powder of minerals3 #ag streak plate3 #ag1 #bb ,streeter3 #dh1 #id striations3 #ha1 #ie strontianite3 #di strontium3 #di sulfur3 #bi1 #cj1 #ca1 #cb1 #cd1 #cf1 #ha1 #ij ,sutton ,county3 #ah1 #ai system3 #b ,t talc3 #bh1 #bi1 #cd1 #hg1 #ic schist3 #hg1 #ic ,talc ,rock3 #ic ,taylor ,county3 #de1 #di1 #fj ,terlingua area3 #df1 #dg1 #di1 #ej1 #fe ,tertiary3 #b1 #c1 #i1 #ba1 #de1 #di1 #ea1 #eb1 #ef1 #eg1 #fj1 #gj1 #gg1 #gh1 #gi1 #hc1 #hd1 #he1 #hf1 #ij1 #ig1 #ih terrazzo-- chips3 #ee1 #gf floors3 #hh tests1 chemical 7see chemical tests7 thorium3 #hj ,thrall oil field3 #hh time1 geologic3 #b1 #c tin3 #dg topaz3 #cf1 #ch1 #id tourmaline3 #cc1 #hb1 #id ,town ,mountain3 #ie ,trans-,pecos area3 #i1 #aj1 #de1 #fj1 #fb1 #fi1 #gh1 #hb1 #hc1 #hd1 #he1 #hg1 #hi1 #ij1 #ih trap rock3 #de travertine3 #df ,travis ,county3 #bj1 #de1 #df1 #di1 #fh1 #fi1 #gj1 #hh tremolite3 #dd1 #ic ,triassic3 #c1 #de1 #hg ,trinity ,county3 #ih tufa1 calcareous3 #df twinned crystals3 #df1 #fc tyuyamunite3 #ig ,u ,upshur ,county3 #gj uranium3 #hj1 #ie1 #if minerals3 #ie uraninite3 #ig uranophane3 #ca1 #ie1 #ig ,uvalde ,county3 #de1 #fj1 #hd1 #hh ,v ,valley ,spring ,gneiss3 #ei ,val ,verde ,county3 #de1 #gd1 #ge ,van ,horn area3 #db1 #eb1 #ec1 #eh1 #ei1 #fj1 #ge1 #gf1 #gi1 #he1 #hg1 #ij ,van ,horn ,mountains3 #de1 #ee1 #gf1 #gi1 #ie ,van ,zandt ,county3 #ff verde antique3 #hh vesicles3 #de vitrophyre3 #dj1 #gg volcanic ash3 #da1 #ig volcanic igneous rocks1 definition of3 #i volcanoes3 #i1 #ig1 #ih ,w ,waco3 #hh wad3 #gc1 #gd ,walker ,county3 #eb ,ward ,county3 #ff ,washington ,county3 #gh1 #hd weathering3 #aa ,webb ,county3 #aa1 #hd ,weches strata3 #gj ,wharton ,county3 #ia white mica 7see muscovite7 ,wilbarger ,county3 #ih ,willacy ,county3 #ff ,williamson ,county3 #bj1 #fj1 #fi1 #gj1 #hh ,willow ,city3 #id wood opal3 #gh wood1 petrified3 #bj1 #eb1 #gh1 #hd ,wylie ,mountains3 #ie ,y yellow copper ore3 #ec ,yoakum ,county3 #ff ,young ,county3 #eh yttrium3 #hj ,z ,zapata ,county3 #hc1 #hd ,zavala ,county3 #hf zirconium3 #hj ,transcriber's ,notes --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,corrected a few palpable typos4 --,included a transcription of the text within some images4 --,in the text versions only1 text in italics is delimited by underscores4 --,added a label to one figure 7to match the ,table of ,illustrations74