Mica
The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having nearly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition. The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.
The word mica is derived from the Latin word mica, meaning a crumb, and probably influenced by micare, to glitter.
Classification
Chemically, micas can be given the general formula
in which
Structurally, micas can be classed as dioctahedral (Y = 4) and trioctahedral (Y = 6). If the X ion is K or Na, the mica is a common mica, whereas if the X ion is Ca, the mica is classed as a brittle mica.
Trioctahedral micas
Common micas:
Biotite
Lepidolite
Muscovite
Phlogopite
Zinnwaldite
Brittle micas:
Clintonite
Interlayer deficient micas
Very fine-grained micas, which typically show more variation in ion and water content, are informally termed "clay micas". They include: