Mica is the name given to a group of silicates differing much from each other in chemical composition and optical properties, but having as a common character an easy cleavage in one direction, and thus affording plates remarkably thin, transparent, tough, flexible, elastic, and pearly. They are mostly silicates of aluminium and potassium, but also contain magnesium, iron, lithium, etc., and a little water. Their hardness ranges from 2 to 3, and their specific gravity from 2-7 to 3-1. They probably all crystallise in the Prismatic system, but occur in six-sided tabular crystals, some of which have clearly two optic axes, whilst others appear uniaxal, the two axes being not recognisably divergent, whence these forms were supposed to belong to the Hexagonal system.
Muscovite, Muscovy glass, common, white or potash mica, is distinctly biaxal, and is a very common mineral, occurring in granite, gneiss, mica-schist (q.v.), and some sandstones. Plates sometimes more than a yard across are obtained in Siberia, Scandinavia, Canada, the United States, and Peru. It is used as a covering for gas-lamps, lanterns, and stoves, or even instead of window-glass. Biotite, black magnesia, or uniaxal mica, named after the French mineralogist Biot, also occurs commonly in igneous rocks. Lepidolite, lithiamica, is a rose-pink or lilac mineral in pearly scales, whence its name (Greek lepis, "a scale"), giving the characteristic lithia red tint to the blowpipe flame. In retail trade muscovite is often erroneously called talc (q.v.).