Calcite, calcium carbonate, is one of the most abundant minerals in
Texas. It is the chief mineral in limestone and in some marble. It also
serves as the cementing material in many sandstones. Crystals, grains,
and cleavable masses of calcite, which have been deposited by
underground water, occur in cracks and cavities in many of the igneous,
metamorphic, and sedimentary rocks of Texas. Calcite also occurs as
cave, spring, and stream deposits and as caliche.
Calcite is transparent or translucent, and—depending on the variety—its
luster is glassy to earthy. Most calcite is white or colorless, but it
can be a shade of pink, blue, green, brown, yellow, or gray. It gives a
white streak when you rub it across a streak plate.
Two properties of calcite to notice are the hardness and the cleavage.
This mineral cleaves perfectly in three directions that are not at right
angles to each other, and some of the cleavage fragments are
rhombohedrons. Calcite is rather soft—you can scratch it with a copper
penny but not with a fingernail. A drop or two of dilute hydrochloric
acid also will help you to identify this mineral. The acid will readily
fizz and bubble when it is placed on calcite.
[Illustration: Calcite has perfect rhombohedral cleavage. The three
directions of cleavage are not at right angles to each other.]
Calcite occurs in more different kinds of crystal shapes than any other
mineral. Some of these crystals are flat and tabular; some are
rhombohedrons; some are prisms. Pointed crystals, called _dog-tooth
spar_, and twinned crystals have been found in the Terlingua area of
Brewster County in west Texas. A somewhat unusual occurrence of calcite
crystals is in geodes. Some of these are found in Lower Cretaceous rocks
west of Austin in Travis County.
Transparent crystals and cleavage fragments of calcite show a property
called _double refraction_ (other minerals show it, too). To test this
property, you can mark a single dot on paper. When you look at the dot
through a piece of clear calcite, you will see two dots instead of one.
This happens because a ray of light is bent (refracted) and is split
into two rays as it enters the mineral. These two rays travel through
the calcite in slightly different directions, and each carries an image
of the dot through the mineral. The two images that you see are at the
points where the two rays leave the calcite.
Calcite that is deposited at springs, along river and creek banks, and
in caves is known as _travertine_. Cave forms of travertine, including
stalagmites and stalactites, occur in several caves in Texas. Another
kind of travertine is called _calcareous tufa_ or _calcareous sinter_.
It is a porous and spongy-looking material deposited from water carrying
dissolved limestone and is found around the openings of some springs and
along some creek and river banks.
A dull, earthy calcite deposit, known as _caliche_, occurs in areas of
Texas that have scant rainfall, such as the High Plains, west Texas, and
the southwestern part of the Gulf Coastal Plain. Caliche commonly is
found mixed with other materials, such as clay, sand, or gravel. This
substance may be firm and compact or loose and powdery.
It is thought that caliche forms when ground moisture, containing
dissolved calcium bicarbonate, moves upward. In dry areas of the
country, this moisture evaporates. As it does, it leaves a crust of
calcium carbonate in the form of caliche on or near the surface of the
ground.
[Illustration: Calcite crystals (dog-tooth spar) from the Terlingua
area of Brewster County, Texas.]
Caliche is quarried in many counties in Texas and is used chiefly as
road material and as an aggregate.
Caliche. _See_ Calcite.
Carnotite. _See_ Uranium Minerals.