|Muscovite - KAl2(SI3Al)O10(OH,F)2.|
The mica group comprises 34 silicate minerals that have a layered or platy texture; large flakes and sheets are found in some metamorphic and igneous rocks. The commercially important micas are muscovite and phlogopite. Mica is stable when exposed to electricity, light, moisture, and extreme temperatures. Mica sheets are chemically inert, dielectric, insulating, lightweight, reflective, refractive, resilient, and transparent to opaque. Sheet muscovite is obtained from coarse-grained igneous rocks called pegmatites. Pegmatites also contain feldspar, quartz, and various accessory minerals. India and Russia are the world's largest producers of sheet mica; the United States production is very small. Sheet, scrap, and flake mica are commercially important. Consumption of sheet mica is as fabricated parts primarily for the electronic and electrical industries. Primary uses for scrap and flake mica are in joint compound, paint, roofing, well drilling additives, and rubber products.
Mica is a mineral name given to a group of minerals that are similar in their physical properties and chemical compositions. They are all silicate minerals, which means that chemically they all contain silica (SiO4). Mineralogists call micas sheet silicates because their molecules combine to form distinct layers. These layers can be seen in muscovite mica specimens because it can be split (mineralogists call this feature cleavage) into very thin, flexible, transparent layers. This physical property is so distinctive that all minerals that cleave in this fashion are said to have micaceous cleavage.
There are 37 different mica minerals. In addition to the silicate tetrahedrons in all micas, Purple micaceous cleavage contains the elements potassium, lithium, and aluminum. Black biotite contains potassium, iron, and magnesium. The two micas used as a commodity are: brown mica or phlogopite which contains iron and magnesium; and the "reddish, green, or white (or clear) mica" or muscovite which contains potassium and aluminum.
Large sheets of muscovite form in igneous rocks. Very large sheets or crystals of muscovite form in a pegmatite. A pegmatite is an extremely slow-cooling igneous rock in which very large crystals can form. Phlogopite generally forms in metamorphic rocks, especially in metamorphosed limestone, although it occasionally forms in igneous rocks, too.
Mica crystals are six-sided. They are fairly light and relatively soft, at 2 to 4 on Moh's harndess scale for the univalent micas. Sheets and flakes of mica are flexible. Mica is heat-resistant and does not conduct electricity.
Two distinct forms of mica are utilized as a commodity. Scrap and flake mica is mica that either occurs naturally or is ground into very small flakes and pieces. Sheet mica is large pieces of mica that can be cut into various shapes for use in electronics.
The name mica was probably created from the Latin word micare meaning to shine in reference to the shiny luster of the micas. Muscovite is very resistant to heat and electricity. As a result, it was commonly called "Muscovy." This mineral was commonly called Muscovy Glass after the Latin term vitrum Muscoviticum. In 1850, James Dwight Dana formally named this mineral muscovite based on the Latin term. The name phlogopite, named by F.A. Breithaupt in 1841, comes from the Greek word phologopos meaning fiery in reference to the reddish color seen on some specimens of this mica.
Scrap and flake mica is produced all over the world. In the U.S., scrap and flake mica was produced in Arizona, North Carolina, South Dakota, Georgia, New Mexico and South Carolina. North Carolina's production accounts for half of total U.S. mica production. The flake mica produced in the U.S. comes from several sources: the metamorphic rock called schist as a by-product of processing feldspar and kaolin resources, from placer deposits, and from pegmatites. Canada, India, Finland, and Japan export flake mica to the U.S.
Sheet mica is considerably less abundant than flake and scrap mica. Sheet mica is occasionally recovered from mining scrap and flake mica. The most important sources of sheet mica are the pegmatite deposits. The United States has limited sheet mica resources. U.S. mining of sheet mica is costly and labor costs are high. As a result, the U.S. imports more than half its sheet mica from India, but also from Belgium, Germany, China, and a few other countries.
The principal use of ground mica is in gypsum wallboard joint compound, where it acts as a filler and extender, provides a smoother consistency, improves workability, and prevents cracking. In the paint industry, ground mica is used as a pigment extender that also facilitates suspension due to its light weight and platy morphology. The ground mica also reduces checking and chalking, prevents shrinkage and shearing of the paint film, provides increased resistance to water penetration and weathering, and brightens the tone of colored pigments. Ground mica also is used in the well-drilling industry as an additive to drilling "muds."
Coarsely ground mica flakes help prevent lost circulation by sealing porous sections of the uncased drill hole. The plastic industry used ground mica as an extender and filler and also as a reinforcing agent. The rubber industry uses ground mica as an inert filler and as a mold lubricant in the manufacture of molded rubber products, including tires.
Sheet mica is used principally in the electronic and electrical industries. The major uses of sheet and block mica are as electrical insulators in electronic equipment, thermal insulation, gauge "glass", windows in stove and kerosene heaters, dielectrics in capacitors, decorative panels in lamps and windows, insulation in electric motors and generator armatures, field coil insulation, and magnet and commutator core insulation. Mica is also used as segment plates between copper commutator sections to insulate copper from the steel; phlogopite mica is used because it wears at the same rate as the copper segments.
Some lightweight mineral and rock materials, such as vermiculite, diatomite and perlite are similar to micas and can be used in place of mica. A long list of manufactured materials, such as styrene, polyester, Teflon, Plexiglass, etc., can be used in place of sheet mica in the electronic applications. Paper made from ground mica can be used in place of sheet mica for insulating applications.