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The Collaborative International Dictionary of English v.0.48 (gcide)

Mica \Mi"ca\, n. [L. mica crumb, grain, particle; cf. F. mica.]
(Min.)
The name of a group of minerals characterized by highly
perfect cleavage, so that they readily separate into very
thin leaves, more or less elastic. They differ widely in
composition, and vary in color from pale brown or yellow to
green or black. The transparent forms are used in lanterns,
the doors of stoves, etc., being popularly called
{isinglass}. Formerly called also {cat-silver}, and
{glimmer}.
[1913 Webster]

Note: The important species of the mica group are:
{muscovite}, common or potash mica, pale brown or
green, often silvery, including {damourite} (also
called {hydromica} and {muscovy glass}); {biotite},
iron-magnesia mica, dark brown, green, or black;
{lepidomelane}, iron, mica, black; {phlogopite},
magnesia mica, colorless, yellow, brown; {lepidolite},
lithia mica, rose-red, lilac.
[1913 Webster] Mica (usually muscovite, also biotite)
is an essential constituent of granite, gneiss, and
mica slate; {biotite} is common in many eruptive rocks;
{phlogopite} in crystalline limestone and serpentine.
[1913 Webster]

{Mica diorite} (Min.), an eruptive rock allied to diorite but
containing mica (biotite) instead of hornblende.

{Mica powder}, a kind of dynamite containing fine scales of
mica.

{Mica schist}, {Mica slate} (Geol.), a schistose rock,
consisting of mica and quartz with, usually, some
feldspar.
[1913 Webster]