Two of the mica minerals that you are most likely to find in Texas are
_muscovite_ and _biotite_. Both these minerals are potassium-aluminum
silicates, and biotite, in addition, contains magnesium and iron. In
general, muscovite is light colored, that is, it has a light brown,
yellow, or green tint, or is colorless, and biotite is dark colored,
commonly dark green, brown, or black. These minerals have glassy or
pearly lusters and are rather soft—a copper penny scratches them. The
specific gravity of biotite is 2.8 to 3.2, and that of muscovite is 2.76
to 3.1.
Mica minerals occur in igneous rocks, such as granite and pegmatite, and
in metamorphic rocks, such as schist and gneiss. They also are found as
tiny flakes in some sandstones, limestones, and other sedimentary rocks.
Most of the Texas mica is found in the Llano uplift area (particularly
in Llano County) and in the Mica Mine area. (The Mica Mine area is in
the Van Horn Mountains about 15 miles south of Van Horn in west Texas.)
In both these areas, the mica minerals occur mostly in Precambrian
pegmatites and mica schists.
The gleaming mica schists were once igneous rocks or sedimentary rocks
such as sandstones and shales. Long ago, great forces beneath the
earth’s surface changed the rocks into mica schists. The mica that is
found in pegmatites formed from hot fluids of igneous origin when the
pegmatite rock itself was formed.
Clusters of mica in the pegmatites are called _books_, because the thin
sheets into which the mica splits look like pages. Some muscovite books
up to 8 inches across are found in the Mica Mine area of the Van Horn
Mountains.
The books or sheets of muscovite mica that occur in pegmatites are
especially valuable to industry. Muscovite can stand great heat without
melting, it is tough, it splits into thin sheets, and it lets very
little heat and electricity pass through. Because of these properties,
muscovite is used in fuses and as insulators in heating elements of
electric irons and toasters. (Biotite is not used, because the iron it
contains makes it a conductor of electricity.) Sheet muscovite also is
widely used by the electronics industry as a non-conducting material in
the manufacture of tubes and other products.
Both muscovite and biotite from mica schist rocks, as well as scrap
pieces of sheet mica from pegmatites, are ground into flakes or powder.
This ground-up mica has many uses, ranging from a powder coating for
automobile inner-tubes to Christmas tree “snow.”
Only a small amount of mica has been mined in Texas. A fair grade of
sheet mica occurs in the pegmatites at Mica Mine in west Texas, but the
deposit is not large. In the pegmatites of the Llano uplift area of
central Texas, no sheet mica has been found that is considered good
enough for the requirements of industry. Mica suitable for grinding,
however, is found in both these Texas areas.
Micaceous Hematite. _See_ Hematite.
Microcline. _See_ Feldspar.
Milky Quartz. _See_ Quartz.
Muscovite. _See_ Mica.
Native Silver. _See_ Silver Minerals.