mica

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of a group of chemically and physically related aluminum silicate minerals, common in igneous and metamorphic rocks, characteristically splitting into flexible sheets used in insulation and electrical equipment.

from The Century Dictionary.

• noun A prefix frequently used in lithology when the rock in question contains more or less mica in addition to the other usual constituents. Thus, mica-syenite, a rock differing very little from ordinary syenite; mica-trap, nearly the same as minette, etc.
• noun One of a group of minerals all of which are characterized by their very perfect bassal cleavage, in consequence of which they can be separated easily into extremely thin, tough, and usually elastic laminæ.
• noun In the preparation of kaolin for use in the manufacture of porcelain, one of the second set of channels through which a mixture of water and suspended clay washed out by the water from the broken clay-bearing rock is slowly passed to obtain the deposition of flakes of mica and other foreign substances, and thus to purify the clay, which is finally allowed to subside in a series of pits or tanks.
• noun A crumb; a little bit.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.
• noun (Min.) an eruptive rock allied to diorite but containing mica (biotite) instead of hornblende.
• noun a kind of dynamite containing fine scales of mica.
• noun (Geol.) a schistose rock, consisting of mica and quartz with, usually, some feldspar.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of a group of hydrous aluminosilicate minerals characterized by highly perfect cleavage, so that they readily separate into very thin leaves, more or less elastic.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of various minerals consisting of hydrous silicates of aluminum or potassium etc. that crystallize in forms that allow perfect cleavage into very thin leaves; used as dielectrics because of their resistance to electricity