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

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.

If you would like to learn more about rocks and minerals in general, the names of several reference books are listed on page 100. In addition, 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 Texas, the United States Geological Survey, and other organizations. A selected list of these reports is given on pages 100-101.

Rocks and minerals are familiar objects to all of us. We pick up attractive or unusual pebbles for our collections, we admire rocky mountain peaks, we speak of the mineral resources of our State and Nation. Rocks and minerals enter, either directly or indirectly, into our daily living. From them come the soils in which grow the grains, the fruits, and the vegetables for our food, the trees for our lumber, and the flowers for our pleasure. The iron, copper, lead, gold, silver, and manganese, the sulfur and salt, the clays and building stones, and the other metals and nonmetals that we require for our way of living were once a part of the earth’s crust.

Earth’s Outer Crust

Rocks and minerals make up most of the outer layer or crust of our earth—the actual ground beneath our feet. The crust is approximately 18 to 30 miles thick beneath the continents. In general, the outermost part consists of many layers of stratified rocks, one above another. The older rocks normally make up the bottom or the deeper layers, and the younger rocks form the upper layers. Not all the layers are perfectly flat and parallel—some are lenticular (lens-shaped), some are tilted, some are partly eroded away, and some are present in one place and absent in another. Beneath the continents, the layers of rock rest on ancient metamorphic rocks and on great masses of igneous rock such as granite. These lower rocks are known as the basement.

Over much of the land surface of the earth, the outermost layer is made up of layers of rock

On the continents, the layers of rock rest on metamorphic rocks and on igneous rocks such as granite

Geologists

Those who study the earth’s crust—its origin, history, rocks, minerals, fossils, and structure—are known as geologists. The geologists who are especially interested in a particular phase of geology, as this science is called, are given special names: those who study fossils are called paleontologists; those who study minerals are called mineralogists; those who study rocks are called petrologists.

Time and Rock Units

The earth’s crust is believed to be at least 3¼ billion years old. In order to deal with this vast stretch of time, geologists have divided the billions of years into various time units and have given each unit a name. The great divisions of geologic time, called eras, are Early Precambrian, Late Precambrian, Paleozoic, Mesozoic, and Cenozoic. These eras are divided into smaller units of time called periods, and the periods are divided into epochs. The [xx time scale] shows the geologic time divisions. Earliest geologic time is shown at the bottom of the scale; most recent is shown at the top.

By examining and studying the different rocks and rock layers, geologists try to discover in which unit of geologic time these rocks formed. Those rocks that formed during a period of geologic time are called a system of rocks; those that formed during an epoch are called a series. For example, the Cambrian System of rocks formed during the Cambrian Period; the Cretaceous System of rocks formed during the Cretaceous Period; the Tertiary System of rocks formed during the Tertiary Period. We are now in the younger epoch (called Recent) of the Quaternary Period of the Cenozoic Era. The rocks that are forming now are the Recent Series of rocks.

ERA

PERIOD

EPOCH

CENOZOIC

QUATERNARY (lasted 0-1 million years)

Recent

Pleistocene

TERTIARY (lasted 62 million years)

Pliocene

Miocene

Oligocene

Eocene

Paleocene

—63 million years ago—

MESOZOIC

CRETACEOUS (lasted 72 million years)

JURASSIC (lasted 46 million years)

TRIASSIC (lasted 49 million years)

—230 million years ago—

PALEOZOIC

PERMIAN (lasted 50 million years)

PENNSYLVANIAN (lasted 30 million years)

MISSISSIPPIAN (lasted 35 million years)

DEVONIAN (lasted 60 million years)

SILURIAN (lasted 20 million years)

ORDOVICIAN (lasted 75 million years)

CAMBRIAN (lasted 100? million years)

—600? million years ago—

LATE PRECAMBRIAN

EARLY PRECAMBRIAN

These time estimates are from the paper, Geologic Time Scale, by J. Lawrence Kulp, published in Science, Vol. 133, No. 3459, April 14, 1961. (The time divisions are not drawn to scale)

EXPLANATION

CENOZOIC

1 Quaternary

2 Tertiary (Oligocene, Miocene, and Pliocene)

3 Tertiary (Eocene)

4 Volcanic (extrusive) igneous rocks

MESOZOIC

5 Upper Cretaceous (Gulf series)

6 Lower Cretaceous (Comanche series)

7 Jurassic

8 Triassic

PALEOZOIC

9 Permian

10 Mississippian and Pennsylvanian

11 Cambrian, Ordovician, Silurian, Devonian and undivided Paleozoic

12 Rocks (Precambrian) older than Paleozoic

13 Intrusive igneous rocks (Precambrian, Mesozoic or Cenozoic)

These rocks are found either at the surface or directly beneath the soils and subsoils which cover most of Texas.

Geologists also subdivide rocks into lesser units. One of these, called a group, is made up of two or more formations. A formation comprises rocks or strata (layers of rock) that are recognized and mapped as a unit. Some formations consist of layers of one particular type of rock, such as limestone or shale. Formations are named after a nearby geographic locality, and in some formation names, the type of rock is included. For example, three of the Texas geologic formations are called Buda Limestone, Del Rio Clay, and Eagle Ford Shale.

Geologic Map

The geologic map (pp. 4-5) shows the rocks that are found at the surface in Texas. Some of these are extremely old. Some, geologically speaking, are very young.