MICA , a See also:group of widely distributed See also:rock-
forming minerals, some of which have important commercial
applications . The See also:principal members of the group are
See also:phlogopite and See also:lepidolite (q.v.) . The name
mica is probably derived from the Latin micare, to shine, to
glitter; the German word glimmer has the same meaning . The See
also:mineral was probably included with selenite under Pliny's
See also:term lapis specularis . Mineralogical Characters.—The
micas are characterized by a very easy cleavage in a single
direction and by the high degree of flexibility, See
also:elasticity and toughness of the extremely thin cleavage
flakes . They all crystallize in the See also:system, often,
however, in forms closely resembling those of the See
also:rhombohedral or orthorhombic systems . Crystals have
usually the See also:form of hexagonal or rhomb-shaped scales,
plates or prisms, with See also:plane angles of 60° and 120°,
and, with the exception of the basal planes, are only rare