Part 15

Granites are now seen at the surface in several areas of Texas. They were gradually uncovered as the areas became higher and the overlying rocks slowly weathered away. One of these areas is the Llano uplift of central Texas where the granites occur in Blanco, Burnet, Gillespie, Llano, and Mason counties. These granites formed during Precambrian time and are believed to be about a billion years old. (Scientists are now able to determine the age of some rocks accurately by very precisely measuring the relative amounts of isotopes produced by decay of radioactive minerals.)

Granites also appear at the surface in the Trans-Pecos country of west Texas. Some of these areas include the Franklin Mountains of El Paso County, the Quitman Mountains of Hudspeth County, the Chisos Mountains of Brewster County, and the Chinati Mountains of Presidio County.

Red, pink, and gray granites from quarries in the Llano uplift area are widely used as building stones and monument stones. A large quarry at Granite Mountain just west of Marble Falls in Burnet County has supplied pink granite for buildings in many parts of the United States. The Texas Capitol building and several other State buildings in Austin are made of this granite.

Graphite

Graphite is a mineral that is made up of a single element—carbon. (Diamond, although it does not look at all like graphite, is a crystalline form of carbon.) Graphite is a steel-gray or black mineral that commonly has a metallic luster. It is not heavy and is extremely soft. Graphite will soil your fingers and leave a black mark on paper. This mineral cleaves perfectly in one direction and splits into thin flakes that feel greasy.

To help distinguish graphite from molybdenite, a mineral it resembles, you can use a shiny, glazed surface, such as is found on a saucer or a plate, to test its streak. When rubbed across this kind of surface, graphite will leave a black streak, but molybdenite will leave a greenish one.

Graphite commonly occurs as scales, as sheet-like layers, or as compact masses. It may be found mixed with clay or other impurities, and it then looks dull and earthy. Crystals of graphite, which are seldom found, are 6-sided and flat.

Graphite occurs in Llano, Burnet, and other counties in the Llano uplift area of central Texas. One of the Nation’s most important graphite mines is located in the Clear Creek area several miles northwest of Burnet in Burnet County. Some graphite has also been mined near Lone Grove in Llano County. In addition, a graphite schist, obtained south of Llano in Llano County, has been used as a filtering material.

All of this graphite occurs in extremely old Precambrian graphite schist rocks that we now see at the surface in this part of Texas. It is believed that the schists were once ancient sedimentary rocks, such as shales, which contained organic matter. Long ago, great forces below the earth’s surface altered these rocks. When this happened, the organic material that they contained changed into the mineral we know as graphite.

Graphite has a number of uses. It is mixed with clay to make the pencil lead that we use for writing. It serves as a lubricant, either alone or mixed with oil, grease, or water. In addition, graphite is used to make generator brushes, stove and shoe polish, and special paints. Because it can stand great heat without melting, some graphite is mixed with clay to make the pots or crucibles that hold molten metals.

Grossularite. See Garnet.

Gypsite. See Gypsum.

Gypsum

Gypsum is a hydrous calcium sulfate. This mineral is normally colorless or white, but impurities cause it to appear gray, brownish, yellowish, or reddish. It is transparent or translucent and is not heavy. When you rub gypsum across a streak plate, it leaves a white streak. This mineral is so soft that a fingernail scratches it easily. Gypsum occurs in several varieties.

The colorless, glassy, and transparent variety of gypsum is called selenite. It is found as cleavable masses and as crystals that are prism-shaped or flat and diamond-shaped. It is not uncommon for two crystals to be joined together so that they have a swallow-tail shape—these crystals are twinned. Groups of flat selenite crystals arranged together so that they resemble flowers are called rosettes. Many of these have been found in Nolan County.

Gypsum has four directions of cleavage. One of these directions is so perfect that some selenite splits into thin, clear sheets that may be mistaken for mica; other selenite cleavage fragments may be mistaken for calcite. You can distinguish selenite sheets from calcite by testing their hardness (selenite is softer) and by putting a drop or two of dilute hydrochloric acid on them. The acid will fizz and bubble on calcite but not on the selenite gypsum. There is also a quick way to distinguish the thin selenite cleavage fragments from mica. After you carefully bend a thin sheet of mica, it will snap back to its original shape without breaking. Selenite gypsum, however, is not elastic. It will bend, but it will break if you try to straighten it again.

Selenite is found in cracks and cavities in rocks. Good crystals have been collected at Gyp Hill, a salt dome southeast of Falfurrias in Brooks County, and some selenite has been mined there. Selenite crystals also occur scattered through clays, particularly along creek banks, in Lee, Fayette, Bastrop, and several other counties.

Another variety of gypsum is known as fibrous gypsum. It is made up of slender, brittle, needle-like fibers that fill the cracks in some rocks. If fibrous gypsum has a silky or pearly luster, it is called satin spar. One of the places where satin spar occurs is in Permian rocks in Hardeman County.

Most of the fibrous gypsum and selenite is formed by solutions. Some of these solutions develop when underground waters, seeping through rocks, pick up and dissolve minerals that contain sulfur (such as pyrite). This dissolved material changes the water into very weak sulfuric acid. When the sulfuric acid meets calcium carbonate (as in limestone or calcite), it combines with the calcium to form the gypsum.

A massive, fine-grained, and translucent variety of gypsum, known as alabaster, is used for articles such as lamp bases, statuettes, vases, and book-ends.

A loose, earthy, crumbly variety of gypsum, called gypsite, is ordinarily found mixed with other materials, such as clay, sand, and soil. It occurs either at or near the surface of the ground. Gypsite is found in Culberson, Reeves, and other counties in west Texas.

A massive, granular variety of gypsum, called rock gypsum, may occur in large deposits. This is the gypsum that is used for making products such as plaster, wallboard, and some cements.

Deposits of rock gypsum are found both underground and at the surface in Texas. Surface deposits occur in Permian rocks in several counties to the east of the Texas High Plains. They also occur in the area between the Pecos River and the Delaware and Apache Mountains in Culberson and Reeves counties. Some of the other surface deposits are found near the Malone Mountains in Hudspeth County and in Lower Cretaceous rocks in Gillespie and Menard counties. Rock gypsum has been mined from the deposits in Fisher, Gillespie, Hardeman, Hudspeth, and Nolan counties. It also has been produced from the cap-rock at Hockley salt dome in Harris County.

Gypsum and another mineral, anhydrite, have very nearly the same composition. Both are calcium sulfates. Gypsum, however, contains water of crystallization, and anhydrite does not. It is likely that most of the rock-gypsum deposits of Texas originally were beds of anhydrite. By absorbing water that seeped through it, the anhydrite changed into gypsum.

Halite

Halite, sodium chloride, is the table salt you sprinkle on food for seasoning. This mineral ordinarily is white or colorless, but other materials cause it to be tinted red, blue, gray, brown, or green. When you rub halite across a streak plate, it leaves a white streak.

Because halite cleaves in three directions, all at right angles to each other, the cleavage fragments are shaped like cubes. You can see some of them by looking at a few grains of table salt through a magnifying glass.

Halite has a salty taste and dissolves easily in water. It also is transparent to translucent and has a glassy luster. This mineral is soft enough for a copper penny to scratch it. Halite commonly occurs as cubic crystals and as granular or compact masses.