Mercury also commonly is used to obtain [gold] from its ores. One method of accomplishing this is to pass wet gold-bearing [gravel] or crushed rock over metal plates that are coated with mercury. The gold particles quickly mix with the mercury to form an amalgam, which later can be scraped off the plates. The gold is then recovered by heating the amalgam to drive off the mercury.
Clay
[Clay] is a smooth, soft, earthy rock made up of mineral particles no bigger than specks of dust. Some of the particles are clay minerals, which consist of aluminum, silicon, and other [elements]. In addition, tiny particles of [quartz], [calcite], and other minerals may also be present in the clay.
The [clay] particles are all that remain of rocks and of minerals, such as [feldspar], that have been broken into fragments or altered into clay minerals by weathering. Some clay remains at the place where it formed, but some is carried away and deposited elsewhere.
[Clay] is white, tan, brown, red, green, blue, gray—almost any color. When moist, it has an earthy odor. You can moisten a piece of clay enough to notice this just by breathing on it. Most clays, when wet, can be molded into many different shapes—that is, they are plastic, but when they are dry, they are firm and solid.
[Clay] is abundant in Texas and has a number of uses. Some goes to make portland cement, and some is baked or burned in a kiln to make brick, tile, sewer pipes, pottery, and other products. This kind of clay is obtained from [Tertiary] [formations] of the [Gulf Coastal Plain], from Upper [Cretaceous] formations in central Texas, and from [Pennsylvanian] formations in north-central Texas. (You can locate Tertiary, Cretaceous, and Pennsylvanian rocks on the Texas [geologic map], pp. [4]-5.)
A special kind of white burning [clay] that can be used to make chinaware is called [kaolin] or china clay. It contains particles of the clay mineral kaolinite as well as several other clay minerals. Deposits of china clay occur in southern Jeff Davis County and in Real County near Leakey, but none is being produced.
Another kind of [clay], [bentonite], forms from weathered [volcanic ash]. Bentonite contains the clay mineral montmorillonite and looks smooth and soap-like. Fresh samples of this clay are white, pale green, or pale blue, but dried-out or weathered samples are tan, brown, yellow, or reddish. When wet, bentonite absorbs water, swells, and then has a jelly-like appearance.
Surface deposits of [bentonite] occur chiefly in [Eocene] [Tertiary] [formations] of the [Gulf Coastal Plain], in [Cretaceous] formations of the Big Bend area of west Texas, and in [Quaternary] formations of the High Plains.
