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.

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); 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. 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.

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.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Hydromica \Hy`dro*mi"ca\, n. [Hydro-, 1 + mica.]

(Min.) A variety of potash mica containing water. It is less elastic than ordinary muscovite.

Hydromica schist (Min.), a mica schist characterized by the presence of hydromica. It often has a silky luster and almost soapy feel.

Source: Webster's Revised Unabridged Dictionary (1913)