biotite

biotite
    n 1: dark brown to black mica found in igneous and metamorphic
         rock
    

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]
    

Biotite \Bi"o*tite\, n. [From Biot, a French naturalist.] (Min.)
   Mica containing iron and magnesia, generally of a black or
   dark green color; -- a common constituent of crystalline
   rocks. See {Mica}.
   [1913 Webster]