mica

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Century Dictionary and Cyclopedia

1. n. A crumb; a little bit.
2. n. 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. They occur in crystals with a prismatic angle of 120, but more commonly in crystalline aggregates, often of large plates, but sometimes of minute scales, having a foliated structure, the folia being generally parallel, but also concentric, wavy, and interwoven, and also arranged in stellate or plumose and sometimes almost fibrous forms. In crystallization the micas belong to the monoclinic system, but they approximate very closely in form in part to the orthorhombic system (e. g., muscovite), in part to the rhombohedral system (e. g., biotite). The micas are silicates of aluminium with other bases, as iron, calcium, magnesium, potassium, sodium, lithium; in some kinds fiuorin is present in small amount. The prominent varieties are—muscovite or common potash mica, the light-colored mica of granite and similar rocks, and paragonite, which is an analogous soda species; biotite, or magnesia mica (including meroxene and anomite, distinguished according to the position of the optic axial plane), the black or dark-green mica of granite, hornblende rocks, etc.; phlogopite, the bronze-colored species common in crystalline limestone and serpentine rocks; lepidomelane, a black mica containing a large amount of iron; and lepidolite, the rose-red or lilac lithia mica occurring commonly in aggregates of scales. (See further under these names.) The micas enter into the composition of many rocks, including the crystalline rocks, both metamorphic and volcanic (as granite, gneiss, mica-schist, trachyte, diorite, etc.), and sedimentary rocks (as shales and sandstones), sometimes giving them a laminated structure. In the sedimentary rocks they are in most cases derived from the disintegration of older crystalline rocks. Mica (muscovite) is often used in thin transparent plates for spectacles to protect the eyes in various mechanical processes, in reflectors, instead of glass in places exposed to heat, as in head-lights and stove- and lantern-lights, and even for windows in Russia (hence called Muscovy glass). Ground to powder, it is combined with varnish to make a glittering coating for wall-papers, and is used also in preparing a covering for roofs, and as a packing and lubricator for machinery. It is often vulgarly called isinglass. The so called brittle micas include a number of species, as margarite, seybertite (clintonite), etc., which are related to the true micas, but are characterized by their brittle folia.
3. n. 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. Each of the first set of channels through which the mixture is passed for the settling of the coarser flakes of mica, etc., is called a drag. This set of channels is collectively called the drags, and the second set the micas. See porcelain and kaolin.
4. n. 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.

Wiktionary

1. n. 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.

GNU Webster's 1913

1. n. (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.

WordNet 3.0

1. n. 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