TEXAS ROCKS AND MINERALS
An Amateur’s Guide
By ROSELLE M. GIRARD
Sketches by Bill M. Harris
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.
Roselle M. Girard
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.
Texas’ highest mountain is Guadalupe Peak, right, with an elevation of 8,751 feet. El Capitan, left, has an elevation of 8,078 feet. These peaks in the Guadalupe Mountains in Culberson County consist largely of Capitan reef limestone, which formed during the Permian Period.
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.
Earth’s outer crust (thickness not drawn to scale).
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.
Geologic time scale
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)
Plate 10. GENERALIZED GEOLOGIC MAP OF TEXAS
Modified from Geologic Map of Texas, 1933
This map in a higher resolution
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.
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.
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Aluminum |
Al |
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Barium |
Ba |
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Beryllium |
Be |
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Boron |
B |
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Calcium |
Ca |
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Carbon |
C |
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Cerium |
Ce |
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Chlorine |
Cl |
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Copper |
Cu |
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Fluorine |
F |
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Gold |
Au |
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Hydrogen |
H |
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Iron |
Fe |
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Lead |
Pb |
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Magnesium |
Mg |
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Manganese |
Mn |
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Mercury |
Hg |
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Molybdenum |
Mo |
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Oxygen |
O |
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Potassium |
K |
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Silicon |
Si |
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Silver |
Ag |
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Sodium |
Na |
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Strontium |
Sr |
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Sulfur |
S |
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Thorium |
Th |
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Tin |
Sn |
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Uranium |
U |
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Vanadium |
V |
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Yttrium |
Y |
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Zinc |
Zn |
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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.
A mineral is made up of chemical elements. The mineral calcite, for example, always consists of the same proportions of 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.
A rock is made up of minerals. The igneous rock granite, for example, consists chiefly of quartz and feldspar; other minerals such as mica and hornblende are commonly present.
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.
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.
Extrusive igneous rocks form at the earth’s surface from lava that cools and hardens relatively quickly.
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.)
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.
Intrusive igneous rocks form from molten rock material (magma) that cools and hardens beneath the earth’s surface.
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.
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.
Soils develop from weathered rock and associated organic material.
SOIL
SUBSOIL
WEATHERED ROCK
BEDROCK
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.
Conglomerate from Webb County, Texas, is composed of rounded gravel that has been cemented together.
Some of the rock fragments carried by water are still fairly large when they reach their destinations. On the basis of size, they are called boulders, cobbles, pebbles, and granules. Loose deposits of these larger-size sediments make up what is known as gravel. Nature cements gravels together to form rocks such as conglomerates (made up of rounded gravel) and breccias (made up of sharp-cornered gravel).
The finer sediments are called sand, silt, mud, and clay. When cemented, the sand grains become sandstones, the silt particles become siltstones, and the mud and clay particles become shale. The sedimentary rocks that are made up of these rock fragments are called clastic or fragmental rocks.
As they are weathered, some rocks dissolve and go into solution. For example, a number of the Texas creeks and rivers carry calcium carbonate in solution because they flow through areas where limestone rocks, which consist mostly of calcium carbonate, are being weathered. (Water that contains a large amount of dissolved rock material is called hard water.)
Some of the waters containing dissolved rock material seep through loose sediments where the dissolved material may come out of solution and form a cement, which binds the sediments together. For example, when loose sand sediments are cemented, they form sandstone. Three of the most common cements are iron oxide, calcium carbonate, and silicon dioxide, although a number of other materials also serve as cements.
Dissolved rock materials come out of solution not only to serve as cementing agents but to form the chief mineral of some sedimentary rocks as well. Sedimentary rocks of this kind form mostly in lakes and seas into which much dissolved material is carried by rivers. When the dissolved material comes out of solution, it is said to be precipitated and the mineral sediments it forms are the chemical sediments. Some limestones originate this way. You can see examples of precipitated materials by noting the crust-like deposits that form inside some water pipes and teakettles, as dissolved material in the water comes out of solution.
Precipitated sediments are commonly observed lining a teakettle.
Sedimentary rocks formed by plants and animals.
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The dissolved rock material can come out of solution in another way. Some plants and animals are able to take dissolved calcium carbonate out of the sea water and use it to build their shells and other structures. Some of these organisms, such as corals and algae, can grow upward from the sea floor in large groups to form reefs that later become reef limestones. Other limestones are made up of the remains of plants and animals that collect on the sea floor and become cemented together.
Metamorphic rocks come from earlier-formed rocks that have undergone a change or a metamorphosis. All igneous and sedimentary rocks, and earlier-formed metamorphic rocks too, can be changed, without being moved to some other place, into new and different rocks. As they are changed, they may become harder, new minerals may form, and they may look entirely different. For example, granite, an igneous rock, can be changed into the metamorphic rock known as gneiss; limestone, a sedimentary rock, can be changed into marble; shale, a sedimentary rock, can be changed into slate. These changes occur because the earth is a big and complex chemical system. The agents that bring about these changes, which always occur below the surface of the earth, are heat, pressure, and fluids—both liquids and gases. Several different kinds of change or metamorphism can take place.
Some of the changes occur because the rocks are at great depths. As more and more younger rocks are deposited on top of them, the older rocks become deeply buried. The great thicknesses of younger rocks are heavy, and they squeeze and press down on the rocks beneath them. The deeply buried rocks are also hotter than surface rocks. In general, the temperature increases about 1° Fahrenheit for each 50 feet of depth below the surface. The change of deeply buried rocks into new rocks by pressure and heat is known as static metamorphism.
Another method of change or metamorphism involves molten igneous rock material. When hot magma moves up through rocks, it not only heats and pushes them, but it also may soak them with liquids and gases, causing the nearby rocks to change into new rocks, by a process called contact metamorphism.
Some rocks are altered by heat and fluids when they are invaded by hot magma in a process called contact metamorphism.
UNALTERED ROCK
METAMORPHIC ROCK
MAGMA
Still another rock-changing process is one that is associated with mountain building. When mountains are formed, heat and great pressures develop deep within the earth’s crust. The flat layers of rock are then slowly pushed and squeezed so that they bend up into arches, fracture, or slide over each other. These forces cause great changes in the rocks in widespread areas. This process of change is known as dynamic metamorphism.
Occurrence and Properties of Minerals
Rocks are made up of minerals. In addition, minerals are associated with rocks in other ways. For example, minerals fill or coat cracks and cavities that have developed in some of the rocks. Minerals are either crystalline or amorphous.
Most minerals are crystalline. In crystalline minerals, combinations of atoms are arranged in ordered patterns, which are repeated over and over. This orderly internal structure of atoms is a characteristic of each crystalline mineral, as mineralogists are able to determine by using X-rays and special microscopes.
Crystals.—
When a mineral occurs as a well-formed individual crystal, it has a definite, precise shape. The kind of crystal shape it has depends on its own type of crystalline internal structure. A well-formed crystal has smooth, flat, outer surfaces called crystal faces, which are arranged together to form prisms, cubes, pyramids, and many other geometric shapes. For example, quartz, a common Texas mineral, is commonly found as a six-sided, prism-shaped crystal that is topped by pyramid-like forms. Pyrite, another common mineral, occurs as cube-shaped crystals. We can identify some minerals more readily by learning to recognize their crystal shapes.
A scalenohedron, one of the many crystal forms of calcite.
A crystalline mineral commonly forms under conditions that do not permit it to become a well-shaped crystal. Although the mineral may show a few crystal faces, it does not have a complete crystal shape and so is described as massive, or is said to occur in masses. Some of the minerals that make up rocks occur as crystalline masses. For example, calcite is a crystalline mineral that occurs in the metamorphic rock marble without its normal crystal shape.
Many crystalline minerals occur as incomplete and imperfect crystals that are grouped together in various arrangements. If these incomplete crystals are arranged around a common center like the spokes of a wheel, they are said to be radial or radiated. If the groups of incomplete crystals look like bundles of strings or fibers, they are described as fibrous. If they are in rounded masses that resemble bunches of grapes, they are called botryoidal. If they look like fish scales, they are described as scaly. Some crystalline minerals are made up of tiny grains that are grouped together like the grains in a lump of sugar. A mineral occurring in this way is described as granular. More descriptions of crystalline minerals are found in the section on Texas rocks and minerals (pp. 43-98).
Barite specimen showing radial form.
An amorphous mineral, unlike a crystalline mineral, does not have a definite, orderly arrangement of its atoms. Because of this lack of internal structure, the mineral occurs in masses that have no regular geometric shapes, and it has no crystal form of its own. Only a few minerals are amorphous.