Mica

mi'ka, a rock-forming mineral well known for its cleavage into thin, elastic laminae or layers. There are several varieties. The best known is a potash mica. It occurs in small gray flakes in granites and gneiss. Masses cleave into remarkably thin, tough plates, having a pearly or silvery surface and a glossy lustre. Mica is transparent and non-combustible. It is used as a substitute for glass and for a variety of purposes. Mica spectacles are used in machine shops to protect the eyes from flying particles that might crack glass. Mica plates are used in headlights and in the construction of lanterns and stoves. Mica is used for windows to such an extent in Russia or Muscovy that it is known to geologists as muscovite. It occurs colorless as well as tinged with brown, green, violet, pink, etc. Black mica owes its color to the presence of iron. Ground to a powder and mixed with varnish, mica makes a glittering ornamental decoration for expensive wall papers. Ground mica is used also as a lubricator for machinery, known in the trade as mica axle grease. Many rocks owe their cleavage to the presence of mica.

The chief source of American mica is the village of Spruce Pine, North Carolina. The mineral occurs in veins in quartz rock. The mica is quarried with care in blocks. The blocks are split with a knife into thin sheets. The sheets are trimmed with shears into various sizes. The largest size known in the regular trade is eight by ten inches. There are 183 sizes and shapes. Mica is worthy from sixty cents to $13 a pound according to size and quality. An armful may be worth several hundred dollars. Scraps and defective blocks are ground into powder.

For an account of a transparent substance of animal origin with which mica is popularly confounded, see ISINGLASS.