Muscovite
Muscovite is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly perfect basal cleavage yielding remarkably thin laminae which are often highly elastic. Sheets of muscovite 5 meters × 3 meters have been found in Nellore, India.
from Paprok, Afghanistan
viewed under cross-polarized light at 2x magnification.
Muscovite has a Mohs hardness of 2–2.25 parallel to the face, 4 perpendicular to the and a specific gravity of 2.76–3. It can be colorless or tinted through grays, browns, greens, yellows, or violet or red, and can be transparent or translucent. It is anisotropic and has high birefringence. Its crystal system is monoclinic. The green, chromium-rich variety is called fuchsite; mariposite is also a chromium-rich type of muscovite.
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 muscovite comes from Muscovy-glass, a name given to the mineral in Elizabethan England due to its use in medieval Russia 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.
