Page [Preface] vii [Introduction] 1 [Earth’s outer crust] 2 [Geologists] 2 [Time and rock units] 2 [Geologic map] 6 [What are rocks and minerals?] 7 [Chemical elements] 7 [Minerals] 7 [Rocks] 8 [Igneous rocks] 9 [Extrusive or volcanic igneous rocks] 9 [Intrusive igneous rocks] 9 [Sedimentary rocks] 10 [Soils] 10 [Sedimentary rock materials in broken fragments] 11 [Sedimentary rock materials in solution] 12 [Cementing materials and chemical sediments] 12 [Sedimentary rocks formed by plants and animals] 12 [Metamorphic rocks] 12 [Static metamorphism] 13 [Contact metamorphism] 13 [Dynamic metamorphism] 14 [Occurrence and properties of minerals] 14 [How minerals occur] 14 [Crystalline minerals] 14 [Crystals] 14 [Imperfect crystals] 14 [Amorphous minerals] 15 [Some distinguishing properties of minerals] 15 [Color] 16 [Luster] 16 [Transmission of light] 16 [Hardness] 16 [Streak or powder] 17 [Cleavage] 17 [Parting] 17 [Fracture] 17 [Specific gravity] 18 [Effervescence in acid] 18 [Some special occurrences of minerals] 18 [Cave deposits] 18 [Concretions] 19 [Geodes] 19 [Petrified wood] 20 [Collecting rocks and minerals] 22 [Rock and mineral identification charts] 24 [How to use the mineral identification charts] 24 [Key to mineral identification charts] 25 [Mineral identification charts] 26 [How to use the rock identification charts] 39 [Rock identification charts] 40 [Descriptions of some Texas rocks and minerals] 43 [Anhydrite] 43 [Asbestos] 43 [Barite] 44 [Basalt] 45 [Calcite] 46 [Cassiterite] 47 [Celestite] 48 [Cinnabar] 49 [Clay] 51 [Copper minerals (chalcocite, chalcopyrite, malachite, azurite)] 52 [Dolomite] 54 [Feldspar] 55 [Fluorite] 56 [Galena] 57 [Garnet] 58 [Gneiss] 59 [Gold] 59 [Granite] 61 [Graphite] 62 [Gypsum] 63 [Halite] 65 [Hematite] 66 [Limestone] 68 [Limonite] 70 [Llanite] 71 [Magnetite] 72 [Manganese minerals (braunite, hollandite, pyrolusite)] 73 [Marble] 75 [Mica] 76 [Obsidian and vitrophyre] 77 [Opal] 78 [Pegmatite] 79 [Pyrite] 80 [Quartz] 81 [Quartzite] 84 [Rhyolite] 85 [Sand and sandstone] 85 [Schist] 87 [Serpentine] 87 [Shale] 88 [Silver minerals (argentite, cerargyrite, native silver)] 89 [Sulfur] 90 [Talc and soapstone] 93 [Topaz] 94 [Tourmaline] 94 [Uranium minerals (carnotite, uranophane, pitchblende)] 95 [Volcanic ash (pumicite)] 97 [Composition, hardness, and specific gravity of some Texas minerals] 99 [Books about rocks and minerals] 100 [Nontechnical books for beginners] 100 [Textbooks and other reference books] 100 [Selected references on Texas rocks and minerals] 100 [Glossary] 102 [Index] 104
Illustrations
Page [Guadalupe Peak and El Capitan in the Guadalupe Mountains, Culberson County, Texas] 1 [Earth’s outer crust] 2 [Geologic time scale] 3 [Generalized geologic map of Texas] 4-5 [A mineral is made up of chemical elements] 7 [A rock is made up of minerals] 8 [Extrusive igneous rocks form at the earth’s surface] 9 [Intrusive igneous rocks form beneath the earth’s surface] 10 [Soils develop from weathered rock and associated organic material] 11 [Conglomerate from Webb County, Texas] 11 [Precipitated sediments lining a teakettle] 12 [Contact metamorphism] 13 [A scalenohedron] 14 [Barite specimen showing radial form] 15 [Chalcedony showing botryoidal form] 16 [Transparent mineral] 16 [Streak plate] 17 [Conchoidal fracture] 18 [Stalactites and stalagmites in the Caverns of Sonora, Sutton County, Texas] 19 [Calcite geode from Travis County, Texas] 20 [Petrified wood from Texas Gulf Coastal Plain] 20 [Prospector’s hammer] 22 [Hand lens] 22 [Physiographic outline map of Texas] 42 [Massive anhydrite] 43 [Amphibole asbestos from Gillespie County, Texas] 44 [Barite cleavage fragment from west Texas] 44 [Basalt from Brewster County, Texas] 45 [Calcite has perfect rhombohedral cleavage] 46 [Calcite crystals (dog-tooth spar) from the Terlingua area of Brewster County, Texas] 47 [Celestite cleavage fragment from Lampasas County, Texas] 48 [Cinnabar and calcite crystals from the Terlingua area of Brewster County, Texas] 50 [Bentonite is used as a drilling-fluid additive] 51 [Hazel copper-silver mine, Culberson County, Texas] 53 [Dolomite rock from Burnet County, Texas] 54 [Feldspar cleavage fragment from Llano County, Texas] 55 [Microcline feldspar crystals from Llano County, Texas] 56 [Fluorite has octahedral cleavage] 57 [Galena has perfect cubic cleavage] 57 [Garnet crystal forms] 58 [Gneiss from Blanco County, Texas] 59 [Placer gold in stream gravels] 60 [Granite from Gillespie County, Texas] 61 [Texas State Capitol building at Austin is made of Burnet County granite] 62 [Graphite is used in pencil lead, generator brushes, and lubricants] 63 [Selenite gypsum crystal from Bastrop County, Texas] 64 [Selenite gypsum rosettes from Nolan County, Texas] 64 [Fibrous gypsum from Terlingua area, Brewster County, Texas] 65 [Salt domes occur on the Gulf Coastal Plain] 66 [Specular hematite from Carrizo Mountains, Hudspeth County, Texas] 67 [Limestone from Travis County, Texas] 68 [Limestone quarry at Georgetown, Williamson County, Texas] 69 [Limonite ore is changed to metallic iron in a blast furnace] 71 [Metallic iron is made into steel in an open-hearth furnace] 72 [Magnetite, Llano County, Texas] 73 [Hollandite from Jeff Davis County, Texas] 74 [Precambrian metamorphic marble from Llano County, Texas] 75 [Mica minerals have perfect cleavage in one direction] 76 [Obsidian arrowheads] 77 [Opalized wood from Washington County, Texas] 78 [Quartz-feldspar pegmatite from Burnet County, Texas] 79 [Pyrite veins in white marble from Llano County, Texas] 80 [Cubic crystals of pyrite] 80 [Quartz crystal from Burnet County, Texas] 81 [Amethyst geode from the Alpine area of Brewster County, Texas] 82 [Milky quartz from Burnet County, Texas] 82 [Smoky-quartz crystals from Burnet County, Texas] 83 [Polished agate from Rio Grande gravels of Zapata County, Texas] 83 [Jasper from Uvalde County, Texas] 84 [Sandstone from Zavala County, Texas] 86 [Prospector] 89 [Sulfur is obtained by the Frasch process] 92 [Talc schist from the Allamoore area of Hudspeth County, Texas] 93 [Topaz crystal from Mason County, Texas] 94 [Black tourmaline crystals with milky quartz from Llano County, Texas] 95 [A Geiger counter is used to detect radioactivity] 96
PREFACE
This booklet has been designed to serve as a brief, simple guide that will be of help to school children, amateur collectors, and others who are just beginning to develop an interest in the rocks and minerals of Texas. It is a companion volume to Texas Fossils by William H. Matthews III published as Guidebook No. 2 by the Bureau of Economic Geology.
Numerous present and former staff members of The University of Texas contributed time and talents to the preparation of this book, and their help is gratefully acknowledged: Peter T. Flawn, Director of the Bureau of Economic Geology, Thomas E. Brown, John W. Dietrich, Alan Humphreys, Elbert A. King, Jr., Peter U. Rodda, and others, including the late John T. Lonsdale, made many helpful suggestions; John S. Harris and Miss Josephine Casey edited the manuscript; Cader A. Shelby prepared a number of the photographs; Bill M. Harris made the illustrative sketches under the direction of James W. Macon; and Cyril Satorsky designed the cover.
Texas Rocks and Minerals
An Amateur’s Guide
Roselle M. Girard
INTRODUCTION
Texas has a great variety of rocks and minerals—some are common and others are not. This book is designed to acquaint you with some of them and to tell you in a nontechnical way what they are like, some of the places where they are found, and how they are used. Although we do not know exactly how all of the rocks and minerals formed, some of the ideas about their origin are mentioned.