Muscovite (also known as Common mica, Isenglass, or Potash mica) is a phyllosilicate mineral of aluminium and potassium with formula: KAl2(AlSi3O10)(F,OH)2. It has a highly perfect basal cleavage yielding remarkably thin laminae (sheets), which are often highly elastic.
Muscovite melts at approximately 1320°C, has a Mohs 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 chromium rich variety is called fuchsite.
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. It is anisotropic, and has quite a high birefringence. Its crystal system is monoclinic.