Serpentine is the name given both to a rock and to a mineral. The
mineral serpentine (a hydrous magnesium silicate) is found in two
different forms. If it is fibrous, it is called _chrysotile_; if it is
layered and platy, it is known as _antigorite_. Antigorite is brownish
green and smooth and waxy looking. Some of it can be split into thin
sheets. Chrysotile is made up of greenish, silky fibers, which may be
brittle and break apart in large pieces. If, however, the fibers can be
pulled apart into soft flexible, little threads, the mineral is called
_chrysotile asbestos_.
Light will pass through both these varieties of serpentine, and both are
soft enough to be scratched by a pocket knife. When rubbed across a
streak plate, they leave white streaks. Antigorite and chrysotile have
no crystal shapes of their own, but several other minerals can alter to
form these two varieties of serpentine. Thus antigorite and chrysotile
may be found as _pseudomorphs_ in a crystal shape that originally
belonged to another mineral.
Antigorite and chrysotile are commonly found closely mixed with
dolomite, talc, magnetite, calcite, pyrite, and several other minerals.
These minerals make up serpentine rock (also called _serpentinite_).
This rock ordinarily is some shade of green (such as whitish, yellowish,
brownish, bluish, or dark blackish green), and it may be mottled. It is
brittle or tough and generally is massive. Serpentine rock, like the
serpentine minerals, is fairly soft—you can scratch it with a pocket
knife.
In the Llano uplift area of central Texas, serpentine rock is found
among Precambrian metamorphic rocks, such as gneiss and schist. An
especially large deposit in this area is known as the Coal Creek
serpentine mass. It is over 3½ miles long, and at one place, it is
almost 1½ miles wide. This mass of serpentine extends across the
Blanco-Gillespie County line in the extreme northern parts of these two
counties. (A little fibrous chrysotile is found here, but it will not
break into flexible enough threads to be called chrysotile asbestos.)
Several other deposits of serpentine occur in northeastern Gillespie
County and in southern Llano County.
It is believed that the Coal Creek serpentine was formed from an igneous
rock such as _peridotite_, which is made up chiefly of grains of the
mineral _olivine_. The peridotite may have been altered into serpentine
by underground waters that seeped through it. It is possible, however,
that other serpentines in the area were formed when rocks were altered
by hot fluids and great pressures far below the earth’s surface.
The Llano area serpentine has been widely used in terrazzo floors. To
make these floors, small pieces of serpentine and other colored rocks
are put into cement that is spread over a concrete slab. Then, after the
cement has hardened, it is ground to a flat, smooth surface and
polished. The resulting terrazzo floor is both colorful and durable.
Serpentine rock also is cut into slabs, polished, and used as indoor
building stones. _Verde antique_, a variety often seen in the lobbies of
office buildings, consists of green serpentine rock with streaks of
white calcite or dolomite in it.
In the Balcones fault zone area (shown on the Texas physiographic
outline map, p. 42) from Uvalde County to Williamson County, serpentine
occurs with Upper Cretaceous rocks. The serpentine rock is seen at the
surface in a few places (such as in Travis and Uvalde counties), but
much of it is underground. In several oil fields of this area (as at
Thrall field in Williamson County and at Lytton Springs field in
Caldwell County), the serpentine rocks contain oil.
Serpentinite. _See_ Serpentine.