Mica is the name of a family of complex aluminum silicate minerals that can be split easily into paper-thin, flexible sheets. If broken across the grain at right angles to the flat, smooth surface, they fracture raggedly. In a single mica crystal, the sheets range from more or less transparent to translucent and are arranged one on top of another like a deck of cards.
Micas are tough and somewhat elastic, soft enough to be split and scratched by a fingernail, and lightweight. They have a nonmetallic, glassy to silky or pearly luster, although yellow mica may appear to be metallic. Color and streak depend upon the chemical composition of the mineral. Muscovite, or white mica (KAl2AlSi3)O10(OH)2), contains potassium and is colorless and transparent in thin sheets. In thick sheets, muscovite varies in color from light yellow to light brown, green, and red and is translucent. It makes a colorless or white streak. Muscovite is named for a region in Russia where large muscovite sheets are used as window panes (muscovy glass). Biotite, also called black mica (K(Mg,Fe)3(AlSi3)O10(OH)2), contains iron and magnesium and is commonly dark green or brown to black, although it may be shades of yellow or brown; its streak is colorless. Thin sheets generally have a smoky color.
Mica is used in electronics, insulators, filler and extender in plasterboard, cement, paint, and drilling mud.