Part 13

Dolomite rock is made up mostly of crystalline grains of the mineral dolomite. In addition, quartz grains, calcite, and other minerals may be present. Dolomite rock is almost any color—white, buff, pink brown, gray. It resembles some limestone, and these two rocks actually are closely related.

To help tell them apart, dilute hydrochloric acid often is used. A few drops of this acid will readily fizz and bubble if the rock you put them on is a limestone. If the rock is dolomite, the acid will effervesce only very little or not at all. (If, however, the acid is put on powdered dolomite, it then will fizz readily.) Dolomite is slightly harder than limestone, and it also is slightly heavier.

Some dolomite rocks formed directly from materials that were dissolved in sea water, and others are altered limestone rocks. Some limestones altered into dolomite on the sea floor by the addition of magnesium from the sea water. Others changed into dolomite much later after the sea had withdrawn and the limestones had become a part of the land; underground waters containing magnesium seeped through these limestones and altered them into dolomite.

Many of the dolomite rocks are found with limestones. In Texas they occur mostly in Cambrian, Ordovician, Mississippian, Pennsylvanian, Permian, and Cretaceous formations. The geologic map (pp. 4-5) indicates where these strata appear at the surface in Texas.

Dolomite is abundant in the Llano uplift area of central Texas—particularly in the Cambrian and Ordovician rocks. A number of these central Texas dolomites have been quarried for use as building stones. Some of them also have been crushed and used as a road-building material and as a stone aggregate that is mixed with cement to make concrete. This dolomite is also used as terrazzo chips (terrazzo floors are described with serpentine on p. 88). In addition, Ellenburger (Ordovician) dolomite from Burnet County was used during World War II as a source of the lightweight metal magnesium.

Dravite. See Tourmaline.

Feldspar

Feldspar is the name given to a group of nonmetallic minerals that are much alike. Several of them are so similar that a petrographic microscope must be used to tell them apart. Each of the feldspar minerals is an aluminum silicate. Each of them contains, in addition, at least one of the following elements: potassium, sodium, calcium, and barium. The feldspar minerals that are found in Texas include albite, a sodium-aluminum silicate, and orthoclase and microcline, which are both potassium-aluminum silicates.

The feldspar minerals are transparent to translucent and have either glassy or pearly lusters. They can be white, cream, or a shade of red, brown, yellow, blue, gray, or green. When you rub a feldspar across a streak plate, it leaves a white streak. The feldspars are rather hard—a pocket knife will not scratch them, although a piece of quartz or a steel file will. These minerals have good cleavage in two directions. The cleavages meet at an angle of about 90°, so that the cleavage fragments have square corners.

The feldspars are important rock-forming minerals. You can find them in igneous rocks, such as granite or pegmatite, and in metamorphic rocks, such as gneiss. They also occur as fragments in sedimentary rocks, such as some sandstone and conglomerate.

Although the feldspars can originate in other ways, they form mostly from hot magmas that cool and crystallize into igneous rocks. These minerals occur in the rocks as grains, as cleavable masses, and as individual crystals. The crystals may be shaped like prisms, or they may be flat and slabby.

Good places to look for feldspars are in areas where granites, pegmatites, and other intrusive igneous rocks appear at the surface. The pegmatite rocks of Burnet, Gillespie, Llano, and Mason counties in the Llano uplift area of central Texas, and those of the Van Horn Mountains in Hudspeth and Culberson counties in west Texas, are especially good sources of feldspar. Large cleavable masses and crystals that are more than a foot long are found in some of these rocks.

The feldspars have a number of uses. Some of the pegmatite feldspars from Llano County in central Texas have been crushed and used as granules for built-up and composition roofs. In addition, some have been shipped to Mexico for glass-making. Some of the other uses of feldspar are in making porcelain, ceramic glazes, and scouring compounds. A few of the feldspar minerals, such as the variety of microcline known as amazonstone, are used as gemstones.

Fibrous Gypsum. See Gypsum.

Flint. See Quartz.

Fluorite

Fluorite is calcium fluoride. The fluorite that is mined and sent to market, however, commonly is found mixed with quartz, calcite, limestone, or other rocks and minerals. Industry calls this mixture fluorspar.

Fluorite is a transparent to translucent mineral that has a glassy luster. It may be colorless, or it may be white, pink, green, purple, brown, or blue. Some specimens show more than one color. When you rub fluorite across a streak plate, it leaves a white streak. This mineral is not particularly hard—a pocket knife will scratch it, although a copper penny will not. Fluorite has perfect cleavage in four directions. By carefully breaking a specimen, you can obtain cleavage fragments that are shaped like octahedrons.

Fluorite occurs as cleavable masses, as fine or coarse grains, and as crystals. Most of the crystals are cubes, but some may be octahedrons, dodecahedrons, or combinations of these.

Fluorite has been found both in west Texas and in central Texas. In the Llano uplift area of central Texas, it occurs in a number of Precambrian granite, pegmatite, schist, and gneiss rocks. The most important, although small, deposit in this area is near Spring Creek a few miles west of Burnet in Burnet County. Here, prospectors have dug holes and pits in gneiss and schist rocks and found layers of fluorite in them.

The largest known fluorite deposits in Texas (they are not particularly large when you compare them with the deposits in Illinois and Kentucky) are those in the Eagle Mountains of Hudspeth County. This fluorite occurs in both igneous and sedimentary rocks. Many years ago, probably during the late part of the Tertiary Period, hot magma far below the surface gave off liquids and gases containing fluorine. These fluids moved up through large cracks (called faults) in Cretaceous limestones and Tertiary igneous rocks and deposited fluorite in them. In places, beds of limestone have been replaced by fluorite. Some of this west Texas fluorite has been mined and shipped to market.

Fluorite is extremely important as a flux in steel-making to help the ingredients of the molten steel blend together. In addition, it combines with sulfur, phosphorus, and other unwanted substances so that they can be removed from the steel. Other important uses of fluorite are in glass-making and in the manufacture of hydrofluoric acid. This acid is used in the aluminum industry as well as in industries that make high-octane gasoline, insecticides, and refrigerants for refrigerators and freezers.

Galena

Galena, lead sulfide, is a shiny, lead-gray, metallic mineral that has a specific gravity of 7.4 to 7.6. It is soft enough to mark paper, and it leaves a grayish-black streak on a streak plate. This mineral cleaves perfectly in three directions, and the cleavage fragments have square corners—some are cubes.

Galena occurs as cleavable masses, as fine or coarse grains, and as crystals, most of which are cubes. Galena commonly is associated with other minerals; for example, some of the west Texas galena either contains some silver (then called argentiferous galena) or occurs with it. Sphalerite, a zinc mineral, is commonly found with galena.

Galena is an important mineral because it is the chief source of lead. Compounds of lead, called white lead, red lead, and litharge, are used as paint pigments. Automobile batteries contain lead plates, and tetraethyl lead is added to gasoline to keep the car’s motor from knocking. Some other uses of lead are in bullets, type metal, solder, and cable coverings.

Galena has been found in several areas of Texas and has been mined in central and west Texas. None, however, has been produced in recent years. Most of the galena mined in west Texas was obtained from silver mines, where the galena was a by-product. Some of the west Texas galena deposits are at Altuda Mountain east of Alpine in Brewster County, in the Eagle Mountains and the Quitman Mountains in Hudspeth County, and in the Chinati Mountains and the Shafter area in Presidio County. Most of the mining has been from the Shafter area (this area is described with silver minerals on p. 90).

In central Texas, several small galena deposits have been found in Blanco, Burnet, and other counties of the Llano uplift area. Some galena has been mined at Silver Creek in northwestern Burnet County. Here, galena occurs in cracks and as scattered grains in Cambrian limestones and sandstones.