Mica, incorrectly called isinglass, is an elastic, fairly soft, platy
mineral, which may be split into flakes of paper thinness. The
relatively clear variety is called muscovite, and the brownish black to
black variety is biotite, both being members of the mica family. They
may occur in Missouri in small grains in the igneous rocks, except that
muscovite may be present in sandstone, where it was deposited along with
the quartz sand.
Mica is used chiefly as insulating material in the electrical industry
where large sheets are required. Another use is as window or chimney
material in stoves or lanterns. Missouri has no mica which is
satisfactory for these purposes.