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 are light weight. They have a
nonmetallic, glassy 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, contains potassium and makes a
colorless or white streak. Biotite, or black mica, contains iron and
magnesium and is commonly dark green or black, although it may be shades
of yellow or brown; its streak is uncolored.
Mica is abundant as tiny, shimmering flakes in Illinois sands,
sandstones, and shales (which are then said to be micaceous). It also is
common in many varieties of igneous and metamorphic rocks. White or
yellow flakes may show a brilliant luster and may be mistaken for
silver, platinum, or gold, but those minerals are heavy and malleable
whereas mica is not.