Produced by Stephen Hutcheson, Dave Morgan and the Online Distributed Proofreading Team at http://www.pgdp.net 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