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Muscovite

A mineral of the mica group with an ideal composition of KAl₂(AlSi₃)O₁₀(OH)₂. Sometimes it is referred to as a white mica or potash mica. Like most micas, muscovite will form "books" of mica crystals from which perfect, thin sheets can be peeled. Muscovite is mined using underground mining. It is found in India, Pakistan, Brazil, and the U.S.

Physical properties include specific gravity 2.76–2.88, hardness on the Mohs scale 2–2.5, and luster vitreous to pearly. Thin sheets are flexible and may be colorless, with books (thick crystals) translucent, yellow, brown, reddish, or green. Muscovite occurs commonly in all the major rock types, in igneous rocks (granites, pegmatites, and hydrothermal alteration products), in metamorphic rocks (slates, phyllites, schists and gneisses), and in sedimentary rocks (sandstones and other clastic rocks). As larger flakes, muscovite is used as an electrical insulator, both for its dielectric properties and for its resistance to heat. Ground muscovite is used for fireproofing, as an additive to paint to provide a sheen and for durability, as a filler, and for many other applications.

See also: Mica;Talc;Dolomite.

The name of muscovite comes from Muscovy-glass, a name formerly used for the mineral because of its use in Russia for windows. It is anisotropic, and has quite a high birefringence. Its crystal system is monoclinic. If you crush this mineral—no matter what color it is—it will form a colorless dust. Light will go through it anywhere from clearly to blurrily. You might be able to see through thin pieces of it. Muscovite would look glassy or pearly if you shined a light on it. If you break this mineral, it will split into sheets or layers.

Material Notes:

One cleavage direction. Occurrence: Igneous, sediments, metamorphic. Very resistant to weathering. Muscovite is used as an insulator because it is heat resistant. It is also used in roofing, cosmetics, and paints. If you compared it to an equal volume of water, it would be about average. If you held it in your hand, it would seem to be about an average weight for its size.

Muscovite is also called white mica. The best way to tell a mica is by the thin, transparent sheets that it cleaves into. Large, clear plates of mica were once commonly used for windows, and the productive Russian mica mines gave muscovite its name. Today mica windows are still used in cast-iron stoves, but the greater use of muscovite is as insulators in electrical equipment.

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