Limestone is a sedimentary rock made up chiefly of calcite, a
calcium-carbonate mineral. This rock also commonly contains grains of
quartz, clay minerals, the mineral dolomite, or other materials. If a
large amount of dolomite is present, the rock is called _dolomitic
limestone_. In some limestones, the mineral grains are too small to be
distinguished from each other without a magnifying glass or a
microscope, but in other limestones, the individual mineral grains are
easily seen.
Pure limestone is white, but if it contains clay or plant or animal
matter it is light gray, dark gray, or black. Limestone also may be some
shade of yellow, brown, or red. It is fairly soft and can be scratched
with a knife. Because this rock contains calcite, an easy chemical test
will help identify it: a drop or two of dilute hydrochloric acid will
quickly fizz and bubble when placed on the limestone.
Limestones form in fresh water, such as in lakes, but most of them form
in the seas. As some earlier-formed rocks are weathered, the calcium
minerals that they contain are dissolved. Creeks and rivers carry this
dissolved material to the sea. There, small animals, such as corals,
crinoids, sponges, and foraminifers, take the dissolved material out of
the water to build their calcium carbonate shells. Plants, such as
algae, can take calcium carbonate out of solution too, and it collects
on them. Shells, shell fragments, and plant remains accumulate on the
sea floor, forming limy deposits that later become limestone.
Limestones also originate in a slightly different way. When the
temperature and chemical composition of the water permit, calcium
carbonate precipitates as millions of tiny grains of calcite and forms a
limy mud that is converted to limestone. Many limestones contain shell
or plant fragments in addition to these tiny grains of calcite.
[Illustration: Polished section of Lower Cretaceous Edwards Limestone
from Travis County, Texas, containing fossil gastropods.]
There are several special kinds of limestone. If the rock is made up of
many little rounded calcite grains that resemble fish eggs, it is called
_öolitic limestone_. Another limestone, _chalk_, is soft, white, and
fine grained. It consists mostly of tiny shell fragments and
fine-grained calcite. _Coquina_ is a porous limestone made up of loosely
cemented shells and shell fragments. Another special kind of limestone,
known as _lithographic limestone_, because it can be used in printing,
is smooth, firm, and hard. Its mineral grains are too small to be
recognized without a microscope. This kind of limestone breaks with a
smooth, sometimes curved, fracture. Still another variety, _pulverulent
limestone_, is loose, soft, powdery, and white. It occurs in the Lower
Cretaceous Edwards Limestone in Williamson and Bell counties of central
Texas. Some of this limestone is used to polish rice grains, and it is
added to livestock feeds to provide calcium for the animals.
[Illustration: Limestone quarry in Lower Cretaceous Edwards Limestone at
Georgetown, Williamson County, Texas.]
Much limestone is found at the surface in Texas in Cambrian, Ordovician,
Mississippian, Pennsylvanian, Permian, and Cretaceous formations. If you
will look at numbers 5, 6, 9, 10, and 11 on the Texas geologic map (pp.
4-5), you will see that these strata appear at the surface in central,
north-central, and Trans-Pecos Texas.
Limestone has many important uses. Much Texas limestone is crushed and
used as a road-building material and as an aggregate that is mixed with
cement to make concrete. Farmers in some areas improve their crops by
adding limestone to the soil. Limestone also is sent to the iron
furnaces in east Texas to be used in the production of pig iron and
steel.
Some of the Texas limestones are heated to a fairly high temperature in
order to change them into _lime_ (calcium oxide). Industry uses a large
amount of lime in making chemicals, steel, glass, paper, and other
products. Builders use it to make plasters, mortars, and stuccos. At
plants in Comal, Johnson, Travis, and Williamson counties, lime is made
from Cretaceous limestones.
Another important use of limestone is in making _portland cement_. The
limestone is mixed with clay or shale, and the mixture is burned in a
kiln until it just begins to melt. Then it is allowed to cool. Next, it
is finely ground and in order to keep the finished cement from hardening
or setting too quickly when it is used, a _retarder_, such as gypsum, is
added. A number of cement-manufacturing plants in Texas use Cretaceous
limestones, shales, and clays.
Many of the Texas limestones make excellent building stones. Some of
them are quarried from Pennsylvanian and Cretaceous formations in
north-central Texas and from Lower Cretaceous formations in counties
near the Llano uplift of central Texas. A large quarry on the
Williamson-Travis County line near Cedar Park in central Texas has
supplied Cretaceous limestone for many buildings and monuments in the
United States and Canada.