Part 10

The mineral grains of some basalts are so small that you cannot distinguish them even with a magnifying glass. Other basalts, however, are porphyritic, which means that they contain larger, easily seen crystals and grains of feldspar and pyroxene scattered either through a mass of the small mineral grains or through glassy material.

Some basalts contain many small holes. These holes, called vesicles, were formed when bubbles of gas were trapped in the hardening magma. Later, solutions moving through the rocks may have deposited another mineral—such as calcite or chalcedony—in some of the vesicles.

Basalt forms from molten rock material that hardens either on or beneath the surface—it can be extrusive or intrusive. Much of the basalt now found in the Trans-Pecos country of west Texas formed from lava that flowed out onto the surface during the Tertiary Period. A few of the places where basalt occurs in west Texas are the Chinati Mountains of Presidio County, the Chisos Mountains of Brewster County, the Davis Mountains of Jeff Davis County, and the Van Horn Mountains of Culberson and Hudspeth counties.

Several varieties of basalt occur in the Balcones fault region of Bandera, Comal, Hays, Kinney, Medina, Travis, and Uvalde counties. These basalts formed from molten magma that forced its way into rocks just below the earth’s surface.

Some basalt, which is known commercially as trap rock, is produced in Uvalde County. It is crushed and used for railroad ballast, road building material, and as concrete aggregate.

Bentonite. See Clay.

Biotite. See Mica.

Braunite. See Manganese Minerals.

Calcite

Calcite, calcium carbonate, is one of the most abundant minerals in Texas. It is the chief mineral in limestone and in some marble. It also serves as the cementing material in many sandstones. Crystals, grains, and cleavable masses of calcite, which have been deposited by underground water, occur in cracks and cavities in many of the igneous, metamorphic, and sedimentary rocks of Texas. Calcite also occurs as cave, spring, and stream deposits and as caliche.

Calcite is transparent or translucent, and—depending on the variety—its luster is glassy to earthy. Most calcite is white or colorless, but it can be a shade of pink, blue, green, brown, yellow, or gray. It gives a white streak when you rub it across a streak plate.

Two properties of calcite to notice are the hardness and the cleavage. This mineral cleaves perfectly in three directions that are not at right angles to each other, and some of the cleavage fragments are rhombohedrons. Calcite is rather soft—you can scratch it with a copper penny but not with a fingernail. A drop or two of dilute hydrochloric acid also will help you to identify this mineral. The acid will readily fizz and bubble when it is placed on calcite.

Calcite occurs in more different kinds of crystal shapes than any other mineral. Some of these crystals are flat and tabular; some are rhombohedrons; some are prisms. Pointed crystals, called dog-tooth spar, and twinned crystals have been found in the Terlingua area of Brewster County in west Texas. A somewhat unusual occurrence of calcite crystals is in geodes. Some of these are found in Lower Cretaceous rocks west of Austin in Travis County.

Transparent crystals and cleavage fragments of calcite show a property called double refraction (other minerals show it, too). To test this property, you can mark a single dot on paper. When you look at the dot through a piece of clear calcite, you will see two dots instead of one. This happens because a ray of light is bent (refracted) and is split into two rays as it enters the mineral. These two rays travel through the calcite in slightly different directions, and each carries an image of the dot through the mineral. The two images that you see are at the points where the two rays leave the calcite.

Calcite that is deposited at springs, along river and creek banks, and in caves is known as travertine. Cave forms of travertine, including stalagmites and stalactites, occur in several caves in Texas. Another kind of travertine is called calcareous tufa or calcareous sinter. It is a porous and spongy-looking material deposited from water carrying dissolved limestone and is found around the openings of some springs and along some creek and river banks.

A dull, earthy calcite deposit, known as caliche, occurs in areas of Texas that have scant rainfall, such as the High Plains, west Texas, and the southwestern part of the Gulf Coastal Plain. Caliche commonly is found mixed with other materials, such as clay, sand, or gravel. This substance may be firm and compact or loose and powdery.

It is thought that caliche forms when ground moisture, containing dissolved calcium bicarbonate, moves upward. In dry areas of the country, this moisture evaporates. As it does, it leaves a crust of calcium carbonate in the form of caliche on or near the surface of the ground.

Caliche is quarried in many counties in Texas and is used chiefly as road material and as an aggregate.

Caliche. See Calcite.

Carnotite. See Uranium Minerals.

Cassiterite

Cassiterite, tin dioxide, is the mineral that serves as the chief source of tin. Tin does not corrode and tarnish, and one of its main uses is in the making of tin cans. (Actually, our tin cans are made from thin sheets of steel that have been coated with a protective layer of tin.)

Cassiterite has either a nonmetallic or a submetallic luster. Some specimens are brilliant and shiny; others are dull. Cassiterite may be translucent to transparent. It may be black, brown, gray, reddish brown, or yellowish brown. When rubbed across a streak plate, this mineral leaves a pale brown, a pale yellow, or a white streak. Cassiterite is quite heavy—it has a specific gravity of 6.8 to 7.1. It is too hard to be scratched by an average pocket knife.

Sometimes, prospectors use a chemical test to help them identify cassiterite. They put small pieces of metallic zinc into a jar or test tube containing dilute hydrochloric acid. Then they add a few fragments of the mineral that they suspect is cassiterite. If the fragments are cassiterite, they become covered with a pale gray coating of metallic tin.

Cassiterite’s most common crystal shape is a short, 8-sided prism with pyramids at each end, but perfect crystals are not often found. Most Texas cassiterite does not show a crystal shape. Instead, it occurs as crystalline masses in igneous rocks and as loose pebbles that have weathered out of these rocks.

Cassiterite occurs in a number of places in the United States but not in large quantities. A small amount of cassiterite has been found in quartz veins in Precambrian granite in both central Texas and west Texas. In El Paso County, the cassiterite is found on the east side of the Franklin Mountains a few miles north of El Paso, where some of it has been mined. In central Texas, cassiterite occurs in the Streeter area of Mason County.

When the granite rocks in these areas were formed, probably not all of the hot magmas cooled and hardened at the same time. The fluids given off by the remaining magmas contained tin and several other elements. It is believed that these fluids moved up into cracks in the granite rocks and formed the cassiterite.

Celestite