Texas Rocks and Minerals: An Amateur's Guide

Hollandite occurs in western Jeff Davis County in west Texas at what is called the Mayfield prospect. Here, it is found as rounded masses that occur in a vein near a large fault in Lower Cretaceous limestone rocks.

Other manganese compounds, pyrolusite and wad, are found in several important deposits near the Pecos River in western Val Verde County. Pyrolusite is a manganese dioxide mineral. It is black, opaque, and so soft that it rubs off on your fingers like soot. Pyrolusite may be granular and massive or may be powdery. It also occurs as a fern-like coating on rocks. Wad is not really a mineral but is an impure, dull-black or brownish-black mixture of manganese oxide, water, and other substances. It can be soft enough to soil your fingers, or it can be too hard to scratch with a pocket knife. Wad occurs in earthy or compact masses or in crusts or stains on rocks.

In Val Verde County, the wad and pyrolusite are found mixed with soil, clay, gravel, sand, and plant remains. This material fills cracks in Lower Cretaceous limestones, it is scattered through gravels, and it is deposited in low places at the surface. The manganese in these deposits came from limestone rocks that have since weathered away. Rainwater trickled into these rocks and dissolved the manganese minerals they contained. This manganese was washed down toward the Pecos River and was deposited as wad and pyrolusite.

Marble

Marble is a metamorphic rock made up chiefly of sparkling grains of calcite or dolomite, but other minerals may be present. The marble may be fine grained, medium grained, or coarse grained; commonly, all the mineral grains are about the same size.

Marble may be of uniform color, banded, spotted, or streaked. If it is made up only of pure calcite or dolomite, the marble is white. If, however, it contains carbonaceous material, such as graphite, it is grayish or black. Limonite impurities cause the marble to be yellowish brown, and manganese oxides and hematite give it a brownish, pinkish, or reddish color.

Marble is a rather soft rock, and you can scratch it easily with a pocket knife. A few drops of dilute hydrochloric acid will bubble and fizz readily on calcite marble; on dolomite marble, it may fizz slightly.

Marble forms from limestone or from dolomite rock. Heat and pressure below the earth’s surface cause the calcite and dolomite mineral grains in these rocks to recrystallize. A fine-grained limestone can be changed into a coarse-grained calcite marble. The marble is not made up of new and different minerals, but it has a new texture unlike that of the limestone. (To a builder, the word marble has another meaning. He considers rocks such as unaltered limestone, unaltered dolomite, or even serpentine to be marble, if they will take a high polish.)

Metamorphic marbles occur at the surface in central Texas and in west Texas. Some of the west Texas occurrences are in the Van Horn area of Culberson and Hudspeth counties and in the Big Bend area of Brewster County. In central Texas, Precambrian marbles are found in Burnet, Gillespie, Llano, and Mason counties of the Llano uplift area. Many of them are suitable for use as monument and building stones. Some of the Llano County marble is quarried and used as granules for roofs and as terrazzo chips for making colorful floors (described with serpentine on p. 88).

Martite. See Hematite.

Mica

Mica is not just one mineral but is the name given to a group of similar minerals. The mica minerals are easy to recognize. Because they have perfect cleavage in one direction, they split into thin, flat sheets. You can see through some mica sheets, and they are elastic enough to be bent back and forth. (Another mineral, selenite gypsum, also will split into thin, flat, transparent sheets, but selenite sheets break when you bend them.)

Basal Cleavage

Two of the mica minerals that you are most likely to find in Texas are muscovite and biotite. Both these minerals are potassium-aluminum silicates, and biotite, in addition, contains magnesium and iron. In general, muscovite is light colored, that is, it has a light brown, yellow, or green tint, or is colorless, and biotite is dark colored, commonly dark green, brown, or black. These minerals have glassy or pearly lusters and are rather soft—a copper penny scratches them. The specific gravity of biotite is 2.8 to 3.2, and that of muscovite is 2.76 to 3.1.

Mica minerals occur in igneous rocks, such as granite and pegmatite, and in metamorphic rocks, such as schist and gneiss. They also are found as tiny flakes in some sandstones, limestones, and other sedimentary rocks. Most of the Texas mica is found in the Llano uplift area (particularly in Llano County) and in the Mica Mine area. (The Mica Mine area is in the Van Horn Mountains about 15 miles south of Van Horn in west Texas.) In both these areas, the mica minerals occur mostly in Precambrian pegmatites and mica schists.

The gleaming mica schists were once igneous rocks or sedimentary rocks such as sandstones and shales. Long ago, great forces beneath the earth’s surface changed the rocks into mica schists. The mica that is found in pegmatites formed from hot fluids of igneous origin when the pegmatite rock itself was formed.

Clusters of mica in the pegmatites are called books, because the thin sheets into which the mica splits look like pages. Some muscovite books up to 8 inches across are found in the Mica Mine area of the Van Horn Mountains.

The books or sheets of muscovite mica that occur in pegmatites are especially valuable to industry. Muscovite can stand great heat without melting, it is tough, it splits into thin sheets, and it lets very little heat and electricity pass through. Because of these properties, muscovite is used in fuses and as insulators in heating elements of electric irons and toasters. (Biotite is not used, because the iron it contains makes it a conductor of electricity.) Sheet muscovite also is widely used by the electronics industry as a non-conducting material in the manufacture of tubes and other products.

Both muscovite and biotite from mica schist rocks, as well as scrap pieces of sheet mica from pegmatites, are ground into flakes or powder. This ground-up mica has many uses, ranging from a powder coating for automobile inner-tubes to Christmas tree “snow.”

Only a small amount of mica has been mined in Texas. A fair grade of sheet mica occurs in the pegmatites at Mica Mine in west Texas, but the deposit is not large. In the pegmatites of the Llano uplift area of central Texas, no sheet mica has been found that is considered good enough for the requirements of industry. Mica suitable for grinding, however, is found in both these Texas areas.

Micaceous Hematite. See Hematite.

Microcline. See Feldspar.

Milky Quartz. See Quartz.

Muscovite. See Mica.

Native Silver. See Silver Minerals.

Obsidian and Vitrophyre

Obsidian is a dark, glassy-looking igneous rock. Most obsidian contains the same chemical elements as granite and rhyolite, since all three of these rocks can form from the same type of molten rock material. Obsidian, however, has no separate minerals, because its chemical elements are not combined in an orderly way. It is a natural glass.

Because it is a glass, we know that obsidian forms very quickly. One way for it to form is from the sudden cooling of hot, molten lava that flows out of volcanoes. If the lava cools and hardens before the separate minerals can crystallize, it becomes a natural glass, such as obsidian.

This rock is smooth and shiny. Most of it is black, but some can be dark green or dark brown. Obsidian allows light to pass through it, and it breaks with a curved, conchoidal fracture. The broken edges are very sharp.

Another glassy igneous rock that forms from fast-cooling lava is vitrophyre. It looks like obsidian except that it has crystals or crystalline mineral grains (which may be light colored) scattered through the dark glassy material.

Obsidian and vitrophyre are found in the Big Bend area of Brewster and Presidio counties in west Texas. They occur with other igneous rocks that formed there during Tertiary time.

The Indians who long ago roamed this area used the smooth, shiny vitrophyre and obsidian to make some of their arrowheads and scrapers. Today, rock collectors pick up these attractive rocks for their collections, and some of them cut and polish obsidian and vitrophyre for use as gemstones.

Onyx. See Quartz.

Öolitic Limestone. See Limestone.

Opal

Opal is like hardened jelly or gelatin. It has no crystalline inner structure and no crystal shape of its own—it is amorphous. This mineral has almost the same chemical composition as quartz. Both are silicon dioxides (silica), but opal, in addition, contains water.

Opal can be almost any color—red, yellow, blue, brown, gray, white—or it can be colorless. It is transparent or translucent and appears glassy, resinous, greasy, or dull. Opal has a specific gravity of 1.9 to 2.2—this mineral is a little lighter than quartz. It also is softer than quartz. A copper penny will not scratch opal, but quartz will. Opal has a white streak and a curved, conchoidal fracture but no cleavage.

Opal occurs in a number of places in Texas. In the Trans-Pecos country of west Texas, it fills cracks and cavities in some of the extrusive igneous rocks. It occurs on the High Plains of northwest Texas, and it is found in Tertiary formations of the Gulf Coastal Plain where it occurs as masses that fill cracks and cavities in sedimentary rocks, as the cementing material in some sandstones (such as in the Catahoula sandstone), and as opalized wood.

Much opal forms from underground waters that contain silicon. These solutions move through the rocks and deposit the opal in them.

Opal is found in a number of varieties. Some show a beautiful, lustrous play of colors that comes from inside the specimens. These varieties are known as precious opal and are prized as gemstones. In Texas, some precious opal is found near Alpine in Brewster County. It has a milky white to bluish-white color, is translucent, and shows a fiery orange, red, blue, and green play of colors.

The variety known as common opal shows no play of colors. It may be white, gray, bluish, reddish, greenish, or yellowish, and it is only slightly translucent. It is found in Brewster, Jeff Davis, Presidio, and other counties of the Trans-Pecos country of west Texas. It occurs also around some of the wet-weather (playa) lakes on the Texas High Plains. In the Gulf Coastal Plain, common opal is found with chalcedony (a variety of quartz) in Tertiary formations. A south Texas locality sometimes visited by collectors is near Freer in Duval County.

A clear, commonly rounded, variety of opal that looks like ice is called hyalite. Two areas in which it has been found are in Presidio County in west Texas and in Llano County in central Texas.

A variety of petrified wood, called opalized wood, is opal that replaced the fibers of a piece of wood. Wood opal is found at a number of places in the Gulf Coastal Plain. It occurs there in Tertiary formations within about 20 miles of the boundary line between areas 2 and 3 shown on the geologic map (pp. 4-5).

A soft opaline material called diatomite, or diatomaceous earth, is made up chiefly of the skeletons of diatoms—tiny, one-celled plants that live in fresh or salt water. These little plants are able to take silica from the water to make opal skeletons for themselves. When the diatom skeletons collect at the bottom of a lake or sea, they form the light, crumbly, white, gray, or cream-colored deposit of impure opal known as diatomite. Industry uses this material as a filter, as insulation, as an abrasive, and as a filler.

Diatomite formed in ancient lakes on the Texas High Plains during late Tertiary (Pliocene) and early Quaternary (Pleistocene) times. It is found in Armstrong, Crosby, Dickens, Ector, Hartley, and Lamb counties.

Opaline Granite. See Llanite.

Orthoclase. See Feldspar.

Pegmatite

Pegmatites occur in igneous rock areas, and most geologists consider them intrusive igneous rocks. They are made up of crystals and crystalline mineral grains that fit together—the grains are interlocked. The crystals and grains in pegmatites are larger than those of surrounding rocks, and some are huge, even larger than a man. However, there is a wide range of grain sizes in pegmatite.

Some pegmatites cut through igneous or metamorphic rocks in such a way that they resemble walls (called dikes). Others are found as veins, as flat masses, or as odd-shaped bodies in rocks. Many pegmatites occur in granites and contain feldspar, quartz, mica, and other minerals, as granite does. Some pegmatites occur with other kinds of igneous rocks and contain the same minerals as these rocks. A few pegmatites contain rare and unusual minerals.

Many geologists believe that pegmatites form from hot fluids of igneous origin that are left after other igneous rocks, such as granite, have already formed. These left-over fluids contain large amounts of aluminum, potassium, silicon, sodium, and several other elements. While the granite or other rocks are still far underground, this material pushes up into them, and may even partly dissolve them. Then it slowly cools and hardens into pegmatite. It is believed that, later, more fluids move into cracks in some pegmatites. This new material adds other minerals to the pegmatites and alters some of those minerals already there.

Some of the pegmatites we now see at the surface in Texas are probably about a billion years old. They formed during Precambrian time and occur with other extremely old rocks. One well-known Texas pegmatite area is the Mica Mine district of west Texas. It is about 15 miles south of Van Horn in the Van Horn Mountains of Culberson and Hudspeth counties. Another pegmatite area is in the Llano uplift of central Texas. These central Texas pegmatites occur in Burnet, Gillespie, Llano, and Mason counties.

Large crystals and grains of feldspar, mica, and quartz are found in the pegmatites of both these areas. A small amount of mica has been mined from the west Texas pegmatites, and feldspar has been produced from the central Texas pegmatites.

An extremely rare and unusual pegmatite occurs in the Llano uplift area at Baringer Hill, which is west of Burnet in Llano County. This pegmatite was once on the bank of the Colorado River, but when Buchanan Dam was built, the area was flooded. The Baringer Hill pegmatite now lies beneath the water of Lake Buchanan. Many rare minerals, which contain beryllium, cerium, thorium, uranium, yttrium, zirconium, and a number of other elements, occur in this pegmatite. Some of these minerals, such as those containing yttrium and zirconium, glow or incandesce when they are heated. During the early part of this century, before the area was flooded, several of the yttrium minerals were mined and used in making lamp mantles.

Pitchblende. See Uranium Minerals.

Precious Opal. See Opal.

Pulverulent Limestone. See Limestone.

Pumicite. See Volcanic Ash.

Pyrite

Pyrite is a shiny, pale golden-yellow or brassy-yellow metallic mineral. This mineral, an iron disulfide, is so often mistaken for gold that it is widely known by the nickname fool’s gold.

Except for their similar color and luster, pyrite and gold are really very different. When you rub pyrite across a streak plate, it leaves a black, a greenish-black, or a brownish-black streak, but the streak of gold is gold-colored. Pyrite is too hard for the average pocket knife to scratch, but a knife will scratch gold easily. Pyrite is brittle and readily breaks, but gold is malleable and flattens out when hit with a hammer. Pyrite is only about 5 times as heavy as an equal volume of water, but pure gold is over 19 times as heavy. And pyrite may have a brown or a multicolored tarnish on it, but gold never tarnishes.

Pyrite is a common mineral and is found in many of the igneous, metamorphic, and sedimentary rocks of Texas. It may be scattered through the rocks, or it may fill cracks and cavities in them. This mineral occurs as granular and compact masses, as rounded masses, or as crystals. The crystals are commonly cubes, pyritohedrons, or octahedrons. In some crystals, the shapes are combined (such as a cube with an octahedron or two pyritohedrons grown through each other). You may notice that the sides of some cubes and pyritohedrons have fine, parallel grooves (called striae or striations) on them.

Pyrite originates in a number of different ways. Some of it forms, along with other minerals in igneous rocks, from hot magmas. It also forms in metamorphic rocks by the same processes that produce these rocks. Some of the pyrite in limestone and other sedimentary rocks is formed when the rocks themselves are deposited by seas or streams. Pyrite also is deposited by the hot fluids that are given off by magmas. These fluids travel up into cracks and other openings in rocks and then form pyrite as well as other minerals. Much pyrite forms in still another way. As water seeps through rocks, it dissolves some of the iron minerals that they contain. When, under certain conditions, these iron solutions mix with hydrogen sulfide (this is the gas that makes some water smell like rotten eggs), pyrite is formed.

Pyrite alters easily. Because of this, most builders carefully check the limestone, granite, marble, or whatever other building stone they plan to use to be sure that it does not contain large amounts of pyrite. When exposed to the weather, pyrite changes to limonite and causes an unsightly rust stain.

Pyrite is used as a source of sulfur, and it is produced for this purpose in several states. In Texas, however, no pyrite deposits have been found that are large enough to be mined.

Pyrolusite. See Manganese Minerals.

Quartz

Quartz, silicon dioxide, is one of the most common minerals. It is glassy, waxy, greasy, or dull and is transparent or translucent. Pure quartz is colorless, but impurities make some varieties white, black, or a shade of red, yellow, blue, violet, or brown. Quartz is a hard mineral. It scratches window glass and cannot be scratched by a pocket knife or even by a steel file. It has a specific gravity of 2.65. The curved, conchoidal fracture shown by many specimens helps identify it.

Quartz is plentiful in Texas. It occurs in igneous rocks, such as granite, llanite, and pegmatite; in metamorphic rocks, such as quartzite, schist, and gneiss; and in sedimentary rocks, such as some sandstone, conglomerate, and breccia.