Talc, a hydrous magnesium silicate, is an extremely soft mineral—your
fingernail scratches it easily. It has a greasy or a pearly luster, and
its color is white, light green, or gray. When rubbed across a streak
plate, it leaves a white streak.
Talc cleaves perfectly in one direction, and the cleavage fragments are
thin, flat, and sheet-like. Its fracture is uneven. This mineral has a
soaplike or greasy feel, and it is sectile—a knife will cut through it.
Talc is not particularly heavy—it has a specific gravity of 2.7 to 2.8.
This mineral seldom occurs with a crystal shape. More commonly it is
massive and is granular or layered.
Talc is not always found as a single, pure mineral. In nature, it
commonly occurs mixed with one or more other minerals, such as
tremolite, anthophyllite, chlorite, and magnetite. This combination of
talc with other minerals forms a soft, greasy or soapy-feeling
metamorphic rock called soapstone. The talc in this rock may be
difficult to identify without special laboratory tests.
In Texas, talc and soapstone are found in Precambrian metamorphic rocks.
In west Texas, talc occurs in an area about 20 miles long (just north of
U. S. Highway 80 in the vicinity of Allamoore, Eagle Flat siding, and
Talc Rock siding) in Hudspeth County. Some of this talc is mined from
open pits and used by the ceramic industry to make wall tile. Some of it
is finely ground, mixed with insect poison, and used as insect powders
and dusts.
[Illustration: Talc schist from the Allamoore area of Hudspeth County,
Texas.]
Deposits of soapstone, containing talc, occur in the Llano uplift area
of central Texas with schist, gneiss, and serpentine rocks in
northeastern Gillespie, northwestern Blanco, and southern Llano
counties. Smaller deposits occur in northeastern Mason County and in
northwestern and southeastern Llano County.
The Llano uplift area soapstones are light green to light buff. It is
thought that some of them were once igneous rocks that contained
magnesium minerals. Fluids, along with great heat and pressures below
the earth’s surface, changed these igneous rocks into soapstone.
Some of this Llano uplift area soapstone is mined from open pits near
Willow City in Gillespie County. It is used mostly in making insect
powders and roofing granules. In addition, some of the central Texas
soapstones have been used for hearths and for fireplace linings.