Gypsum is a hydrous calcium sulfate. This mineral is normally colorless
or white, but impurities cause it to appear gray, brownish, yellowish,
or reddish. It is transparent or translucent and is not heavy. When you
rub gypsum across a streak plate, it leaves a white streak. This mineral
is so soft that a fingernail scratches it easily. Gypsum occurs in
several varieties.
The colorless, glassy, and transparent variety of gypsum is called
_selenite_. It is found as cleavable masses and as crystals that are
prism-shaped or flat and diamond-shaped. It is not uncommon for two
crystals to be joined together so that they have a swallow-tail
shape—these crystals are _twinned_. Groups of flat selenite crystals
arranged together so that they resemble flowers are called _rosettes_.
Many of these have been found in Nolan County.
Gypsum has four directions of cleavage. One of these directions is so
perfect that some selenite splits into thin, clear sheets that may be
mistaken for mica; other selenite cleavage fragments may be mistaken for
calcite. You can distinguish selenite sheets from calcite by testing
their hardness (selenite is softer) and by putting a drop or two of
dilute hydrochloric acid on them. The acid will fizz and bubble on
calcite but not on the selenite gypsum. There is also a quick way to
distinguish the thin selenite cleavage fragments from mica. After you
carefully bend a thin sheet of mica, it will snap back to its original
shape without breaking. Selenite gypsum, however, is not elastic. It
will bend, but it will break if you try to straighten it again.
[Illustration: Selenite gypsum crystal from Bastrop County, Texas.]
Selenite is found in cracks and cavities in rocks. Good crystals have
been collected at Gyp Hill, a salt dome southeast of Falfurrias in
Brooks County, and some selenite has been mined there. Selenite crystals
also occur scattered through clays, particularly along creek banks, in
Lee, Fayette, Bastrop, and several other counties.
Another variety of gypsum is known as _fibrous gypsum_. It is made up of
slender, brittle, needle-like fibers that fill the cracks in some rocks.
If fibrous gypsum has a silky or pearly luster, it is called _satin
spar_. One of the places where satin spar occurs is in Permian rocks in
Hardeman County.
[Illustration: Selenite gypsum rosettes from Nolan County, Texas.]
Most of the fibrous gypsum and selenite is formed by solutions. Some of
these solutions develop when underground waters, seeping through rocks,
pick up and dissolve minerals that contain sulfur (such as pyrite). This
dissolved material changes the water into very weak sulfuric acid. When
the sulfuric acid meets calcium carbonate (as in limestone or calcite),
it combines with the calcium to form the gypsum.
[Illustration: Fibrous gypsum from Terlingua area, Brewster County,
Texas.]
A massive, fine-grained, and translucent variety of gypsum, known as
_alabaster_, is used for articles such as lamp bases, statuettes, vases,
and book-ends.
A loose, earthy, crumbly variety of gypsum, called _gypsite_, is
ordinarily found mixed with other materials, such as clay, sand, and
soil. It occurs either at or near the surface of the ground. Gypsite is
found in Culberson, Reeves, and other counties in west Texas.
A massive, granular variety of gypsum, called _rock gypsum_, may occur
in large deposits. This is the gypsum that is used for making products
such as plaster, wallboard, and some cements.
Deposits of rock gypsum are found both underground and at the surface in
Texas. Surface deposits occur in Permian rocks in several counties to
the east of the Texas High Plains. They also occur in the area between
the Pecos River and the Delaware and Apache Mountains in Culberson and
Reeves counties. Some of the other surface deposits are found near the
Malone Mountains in Hudspeth County and in Lower Cretaceous rocks in
Gillespie and Menard counties. Rock gypsum has been mined from the
deposits in Fisher, Gillespie, Hardeman, Hudspeth, and Nolan counties.
It also has been produced from the cap-rock at Hockley salt dome in
Harris County.
Gypsum and another mineral, anhydrite, have very nearly the same
composition. Both are calcium sulfates. Gypsum, however, contains water
of crystallization, and anhydrite does not. It is likely that most of
the rock-gypsum deposits of Texas originally were beds of anhydrite. By
absorbing water that seeped through it, the anhydrite changed into
gypsum.