Chapter I: Front Matter

BUREAU OF ECONOMIC GEOLOGY
The University of Texas
Austin, Texas
Peter T. Flawn, Director

Guidebook 6

TEXAS ROCKS AND MINERALS
_An Amateur’s Guide_

_By_
ROSELLE M. GIRARD

_Sketches by Bill M. Harris_

February 1964
Second Printing, April 1972
Third Printing, April 1976
Fourth Printing, May 1979

Contents

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