KAl2AlSi3O10OH2

KAl2AlSi3O10OH2 is the empirical formula commonly associated with muscovite, a potassium aluminum silicate hydroxide mineral in the mica group. In mineralogical notation its idealized formula is KAl2(AlSi3O10)(OH)2. Muscovite is a sheet silicate (phyllosilicate) with a characteristic 2:1 layering: two tetrahedral silica sheets bound to a central octahedral sheet, with potassium ions between layers that separate the sheets. This structure gives the mineral its perfect basal cleavage and its distinctive flexible, sheet-like crystals.

Physical and optical properties commonly observed include color ranges from colorless to pale yellow, brown, or green; transparent to translucent material; a vitreous luster; and a hardness of about 2 to 2.5 on the Mohs scale. The streak is white, and the mineral exhibits strong pleochroism in some varieties. Muscovite crystals are monoclinic and typically form tabular crystals capable of being split into thin, transparent sheets along the (001) plane.

Occurrence and formation occur in igneous rocks such as granites and pegmatites, as well as in metamorphic rocks resulting from high-temperature, low-pressure conditions. It is widely distributed in the continental crust and is commonly associated with other micas and potassium-bearing minerals. Notable localities include parts of Europe, North America, and Asia.

Applications and significance include historic use as an electrical insulator and dielectric in equipment, owing to its high resistance to heat and electricity. Muscovite is also used as a filler and extender in paints, plastics, and cosmetics, and it serves as an indicator mineral in geological exploration for potassium-rich systems. The name muscovite derives from its historic association with glass substitutes from Muscovy (Russia).