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].