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.
[Illustration: Bentonite is used as a drilling-fluid additive in the
rotary method of drilling for petroleum and gas.]
Some bentonite is used to absorb unwanted coloring material in petroleum
and in vegetable oils. It is then known as a _bleaching clay_. Bentonite
bleaching clay is obtained from some of the Tertiary formations along
the Texas Gulf Coastal Plain. It has been produced in Angelina, Fayette,
Gonzales, Jasper, Walker, and other counties in this area.
Another important use of bentonite, and of other clay, too, is as
_drilling mud_. In the rotary method of drilling for oil and gas, mud is
pumped down into the drilled hole. This mud carries the rock cuttings up
to the surface, it cools the drilling tools, and it coats and seals the
walls of the hole. Along the Gulf Coastal Plain, drilling clay is
obtained from Tertiary formations.
Common Opal. _See_ Opal.