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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

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.

WHAT ARE ROCKS AND MINERALS?

Although _rocks_ and _minerals_ are often mentioned together, and to some people they have similar meanings, geologists make a distinction between the two words. In general, rocks are made up of minerals, and minerals are made up of chemical elements.

Chemical Elements

The _chemical elements_ include oxygen, silicon, calcium, sulfur, carbon, gold, silver, and many others. There are 90 naturally occurring elements. Each is made up of molecules that consist of only one kind of atom. Chemical elements may either be combined with each other or occur alone. They are the building blocks of our world for they make up all the gases, all the liquids, all the minerals, all the plant and animal life, and all the other physical matter. Some of the chemical elements that occur in the rocks and minerals mentioned in this book are listed below.

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 alphabet. The letters, like the chemical elements, are fundamental building blocks, and they can be brought together in various combinations to form words.

Minerals

A _mineral_ can be compared to a word of our language. We combine letters to form a word, and nature combines certain chemical elements to form each particular mineral. For example, calcite, a mineral that is abundant in Texas, is always made up of the same proportions of the same three elements: calcium, carbon, and oxygen.

Each mineral has its own characteristic internal structure and other properties. At ordinary temperatures, nearly all the minerals are solids rather than gases or liquids. (Water and mercury are the principal exceptions.) In addition, minerals are inorganic rather than being composed of plant or animal matter.

When a single chemical element is found alone in nature as a _solid_, it is considered to be a mineral, too. Gold, silver, copper, lead, and sulfur are some of the chemical elements that can occur alone as solid minerals. When they occur this way, we refer to them as _native_ silver, _native_ copper, or _native_ sulfur. Although the element mercury is a liquid rather than a solid at ordinary temperatures, it too is a mineral when it occurs alone in nature. It is then called _native_ mercury.

Rocks

We have already compared the chemical elements to the alphabet and the minerals to words. We can now go a step further and compare rocks to sentences. We put words together to make sentences; nature puts minerals together to make rocks. A sentence does not have to be made up of a definite number of words, nor does a rock have to be made up of a definite number of minerals. Some rocks, such as granite, may be composed of several minerals. Others, such as dolomite and rock gypsum, consist of only one mineral.

Minerals do not lose their identities when they make up a rock. Instead, they are merely associated together in varying proportions. Some rocks, as we will find later, instead of being composed of the minerals themselves, are made up of fragments of earlier-formed rocks.

Ordinarily, we think of rocks as hard and solid substances, such as limestone and granite, but some geologists consider loose and uncemented materials, such as sand, gravel, or volcanic ash, to be rocks also. The words _sediments_ or _deposits_ are often used to describe this uncemented or loose material.

Rocks are commonly grouped, according to how they formed, into three great classes known as _igneous_, _metamorphic_, and _sedimentary_.

IGNEOUS ROCKS

Igneous rocks result from the cooling of hot, molten rock material or _magma_. Magma that reaches the surface through volcanoes is called _lava_. Magma comes from deep within the earth and is made up of a mixture of molten mineral materials. Igneous rocks have been forming throughout the geologic past and are still forming today. We can understand how they form when we look at pictures of hot, molten lava flowing from volcanoes, such as Mauna Loa in Hawaii. As lava cools, it hardens into rock.

Extrusive or Volcanic Igneous Rocks

The igneous rocks that form on the earth’s surface are called _extrusive_ or _volcanic_ igneous rocks. When magma flows to the surface, it cools and hardens quickly. The mineral grains that form during this fast cooling may be too small to be distinguished from each other. Some lava cools too quickly for minerals to crystallize—then the rock is volcanic glass.

No volcanic igneous rocks are forming in Texas now. However, during Tertiary time, in the Big Bend area and in other parts of the Trans-Pecos country of west Texas, lava came to the surface and hardened. (The physiographic outline map, p. 42, shows where these areas are located.)

Intrusive Igneous Rocks

The cooling and hardening of hot, molten magma also takes place below the earth’s surface. Here, the magma cools slowly to form rocks made up of mineral grains that are large enough to be readily visible. These rocks are known as _intrusive_ igneous rocks. We know that they are present below the surface in Texas because of wells drilled in many areas of the State. In Pecos County, a well reached granite, an intrusive igneous rock, at a depth of 16,510 feet. Other wells in Texas have reached the granite basement rocks at much shallower depths. But not all intrusive igneous rocks in Texas are found underground. In the Trans-Pecos country of west Texas, in the Balcones fault zone, and in the Llano uplift of central Texas, some are now seen at the surface. They, like all intrusive rocks, were formed below the ground, but earth’s processes of uplift and erosion have gradually uncovered them.

SEDIMENTARY ROCKS

Sedimentary rocks are made up of sediments, which are rock and mineral grains that have come from weathered rocks of all kinds. Rocks are weathered when water, ice, snow, wind, and other agents cause them either to dissolve, as table salt does when put in water, or to break apart, as old pavement commonly does.

Soils

Some of the broken-down rocks, along with associated plant and animal matter, develop into soils. When you examine soil with a magnifying glass, you may be able to see some of the small rock and mineral grains that still remain in it. Some soils have formed on top of the rocks from which they came, and some have been moved in from another place.

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 (the sediments) and deposit them in other places. Whenever you see a dust or sand storm, or a muddy creek or river, you are observing the movement of sediments by wind and water to other land areas or to the sea. The combination of weathering and movement is called _erosion_.