Muscovite

Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated 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 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, transparent or translucent. It is anisotropic and has high birefringence. Its crystal system is monoclinic. The green chromium-rich variety called fuchsite. Mariposite is also a chromium-rich type of muscovite.

The name comes from Muscovy-glass, a name given to the mineral in Elizabethan England due to its use in medieval Russia (Muscovy) as a cheaper alternative to glass in windows. This usage became widely known in England during the sixteenth century with its first mention appearing in letters by George Turberville, the secretary of England’s ambassador to the Muscovite tsar Ivan the Terrible, in 1568.