The Collaborative International Dictionary of English v.0.48:

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]