Muscovite, also known as potash mica, is a phyllosilicate mineral of aluminium and potassium. It has a highly perfect basal cleavage yielding remarkably thin laminae (sheets), which are often highly elastic. Muscovite has a hardness of 2 – 2.25 and a specific gravity of 2.76 – 3. It can be colorless or tinted through grays, browns, greens, yellows, or rarely violet or red, and can be transparent or translucent. The green form is called Fuchsite, KAl2(AlSi3O10)(F,OH)2.
Muscovite is the most common mica, found in granites, pegmatites, gneisses and schists, and as a contact metamorphic rock or as a secondary mineral resulting from the alteration of topaz, feldspar, kyanite, etc. In pegmatites, it is often found in immense sheets that are commercially valuable. Muscovite is in demand for the manufacture of fireproofing and insulating materials and to some extent as a lubricant.
The name of muscovite comes from Muscovy-glass, a name formerly used for the mineral because of its use in Russia for windows.