Part 14

It is probable that much of the galena in west Texas and in central Texas was formed when hot magma forced out solutions containing lead. These solutions moved up through cracks and other openings in the subsurface rocks and deposited the galena in them.

Small amounts of galena, which likely had a different origin, have been found in Fisher, Foard, Hardeman, and Young counties. A little occurs also in rocks associated with salt in a number of the Gulf Coastal Plain salt domes.

Garnet

Garnet is not one mineral but is the name given to a group of several minerals that are very much alike. In fact, it often is impossible to tell some of them apart without using special laboratory tests.

The garnet minerals have glassy to resinous lusters and are transparent or translucent. A pocket knife will not scratch them, and some specimens are too hard even for quartz to scratch. Two of the garnet minerals most commonly found in Texas are almandite, an iron-aluminum silicate, and grossularite, a calcium-aluminium silicate. Almandite has a deep-red or a brownish-red color. Grossularite is pale green, brownish yellow, cinnamon brown, or rose red.

Garnet minerals occur as crystals and as masses that are scattered through some of the metamorphic and igneous rocks. After they have weathered out of these rocks, the garnets make up a part of many sands and sandstones. Because these minerals so commonly occur as crystals, it is helpful to learn to recognize the crystal shapes.

Garnet minerals are found in the igneous and metamorphic rocks of both central Texas and west Texas. In central Texas, they occur in ancient Precambrian schist and pegmatite rocks of the Llano uplift area. Some of these central Texas garnet localities are in northeastern Mason County, central and northwestern Llano County, west-central Burnet County, and northeastern Gillespie County.

In west Texas, garnets occur in metamorphic rocks in the Quitman Mountains, which are southwest of Sierra Blanca in Hudspeth County, and in the Mica Mine area, which is south of Van Horn near the Hudspeth-Culberson County line. Garnets also have been found in igneous rocks in the Franklin Mountains a few miles north of El Paso in El Paso County.

Garnets that are found in metamorphic rocks such as schists were formed when great forces squeezed and heated rocks far below the earth’s surface. This heat and pressure caused elements in the rocks to join together into different combinations to form new minerals, such as garnets. Garnets that occur scattered through igneous rocks, such as some pegmatites and granites, cooled and crystallized from hot, igneous fluids when the rocks themselves formed.

Most Texas garnets are not transparent. A few, however, are clear enough to be used as gemstones. These can be cut, polished, and mounted in rings, brooches, bracelets, and earrings. Although some garnet is widely used as an abrasive, none from Texas has been produced for this purpose.

Gneiss

Gneiss is a metamorphic rock that has parallel layers or bands. Some gneiss is made up of the same minerals (chiefly feldspar and quartz) as granite, and it is then called granite gneiss. Several of the other kinds of gneiss are known as mica gneiss, conglomerate gneiss, gabbro gneiss, and hornblende gneiss. In order to be a gneiss, a metamorphic rock has to have bands or layers. These bands may be either straight or wavy and either wide or narrow. In most gneisses, you will find a layer made up of long or flat mineral grains next to a layer made up of the grains of an entirely different mineral. The bands may show color differences, too. For example, a pink layer made up of feldspar grains may be found next to a black layer made up of hornblende grains. The mineral grains interlock as they do in igneous rocks, and they are generally large enough to be seen without a magnifying glass.

Gneiss can form from an igneous rock, such as granite, or from a sedimentary rock, such as sandstone. Heat, fluids, and pressures below the earth’s surface change these rocks into gneiss.

Gneiss that formed during Precambrian time is now seen at the surface in both west Texas and central Texas. In west Texas, it occurs principally in the Van Horn area of Culberson and Hudspeth counties. In central Texas, it is found in Blanco, Burnet, Gillespie, Llano, and Mason counties of the Llano uplift area.

One of the Llano uplift rocks is called the Valley Spring Gneiss. It generally has a light color (much of it is pinkish), and it is believed to have once been a sandstone. Another gneiss of this area, the Big Branch Gneiss, which has a medium to dark gray color, occurs in northern Gillespie and Blanco counties and is an altered igneous rock. Some of the Texas gneiss rocks are suitable for use as building stones.

Gold

Gold commonly occurs in nature as a single element—gold—but much native gold has a small amount of some other element, such as silver, copper, or iron, mixed with it.

Native gold is a shiny, yellow, metallic mineral that does not tarnish, and it leaves a shiny, golden-yellow streak when you rub it across a streak plate. If silver is present, the color and streak have a lighter shade. Pure gold is extremely heavy—its specific gravity is 19.3. Because it is malleable, this mineral will flatten into a thin sheet when hammered. It is ductile enough to be drawn out into wires. Gold is also soft—a pocket knife will scratch it easily. When it is to be used for ornaments and jewelry, gold is usually mixed with other metal, such as silver, copper, nickel, or palladium, to make it harder. The amount of gold that is present is then indicated by carats (or karats). Pure gold is 24 carats. If you have a gold ring that has 14 K stamped inside it, you know that it is made of a mixture of 14 parts gold and 10 parts of other metal.

Gold commonly occurs in nature as plates, scales, or grains. Some of the grains are large enough to be called nuggets. It also is found in a wire-like shape described as filiform, it occurs in a network, called reticulate, and it can have a branching and fern-like shape, described as dendritic. Gold is not often found as individual crystals.

Several other minerals, such as pyrite, chalcopyrite, and mica, are sometimes mistaken for gold. None of these, however, is malleable and ductile, and none is nearly as heavy as gold. Pyrite and chalcopyrite have dark-colored streaks unlike that of gold. Mica cleaves so perfectly that it can be split into thin, flat sheets, but gold has no cleavage at all.

The best places to look for gold are in areas near igneous rocks and along the creeks and rivers that drain these areas. It is thought that most gold originally was carried up from molten igneous rock by hot solutions. The solutions moved into cracks and other openings in nearby rocks and deposited the gold, commonly along with quartz. Later, some of these gold-bearing rocks weathered away. The gold that the rocks contained either remained at the spot or was washed into creeks and rivers. These transported accumulations of loose gold are called placer deposits.

No really important gold deposit has ever been found in Texas, although traces and small amounts have been reported in several areas. A little gold has been found in the Llano uplift area of central Texas. It occurs in quartz veinlets that cut through some of the Precambrian metamorphic rocks of Llano, Mason, northeastern Gillespie, and west-central Burnet counties. Many years ago, a small amount of gold was mined northeast of Llano in Llano County from the Heath mine. Some gold also has been found in sands and gravels along streams, such as along Sandy Creek and its tributaries, in parts of this Llano uplift area.

In the Trans-Pecos country of west Texas, small amounts of gold have been found in the Van Horn area of Culberson and Hudspeth counties, in the Quitman Mountains district of Hudspeth County, and in the country around Shafter in Presidio County. Most of the small quantity of gold that was mined in west Texas was obtained as a by-product from the Presidio mine in the Shafter district (described with silver minerals on p. 90).

Small amounts of gold have been reported from other parts of Texas. Some of these localities are in Eocene Tertiary sandstones in the Gulf Coastal Plain, in Cretaceous limestones in Irion, Uvalde, and Williamson counties, and in sand and gravel in Howard and Taylor counties. None of these deposits has been found to have any commercial value.

Granite

Granite is an intrusive igneous rock that is made up chiefly of crystalline grains or crystals of quartz and a feldspar mineral, such as orthoclase or microcline. Several other minerals, including mica and hornblende, may also be present.

All of the mineral grains in granite are about the same size, and you can distinguish them without using a magnifying glass. A granite may be coarse grained, medium grained, or fine grained. When you examine this rock, you will see that its grains are not cemented but interlocked like the pieces of a jigsaw puzzle. The color of granite, which is pink, red, gray, or brownish, depends chiefly on the color of its feldspar grains.

Most granites formed from hot, molten magma that slowly cooled and hardened far below the earth’s surface. Because of this slow cooling, fairly large mineral grains were formed.