Mica
Mica, any of several rock-forming minerals. Micas have a pearly or glassy luster and occur in the form of flakes, scaly masses, or six-sided crystals. The crystals can be split into very tough, flexible sheets less than 1100 of an inch (14 mm) thick.
Micas are composed essentially of hydrous potassium-aluminum silicate. Some also contain magnesium, iron, lithium, or other elements. There are four major kinds of mica:
Biotite, or Black Mica,is the most common. It is usually black, dark brown, or dark green and is found in gneiss, schist, and many kinds of igneous rocks. Biotite is the most common source of vermiculite, a group of minerals formed in nature from mica and water.
Muscovite, or White Mica,is commercially the most important. Muscovite crystals are translucent and pale brown, green, or gray. Sheets of muscovite are usually transparent and colorless. Muscovite occurs in gneiss, schist, and pegmatite, a coarse-grained igneous rock.
Phlogopite, or Amber Mica,is pale yellow to pale brown. Phlogopite is found in limestone, serpentine, and some kinds of igneous rock.
Lepidolite, or Lithia Mica,is relatively rare and occurs chiefly in the form of scaly masses. Lepidolite is lavender or pink and is found in pegmatites.
Muscovite and phlogopite have properties that give them several important industrial uses—they are very poor conductors of electricity and heat, and they split into thin sheets. Lepidolite is an important source of lithium and rubidium. Biotite has no commercial uses.
Sheet mica is used for electrical insulation in electron tubes, capacitors, electric motors, and other electrical apparatus. Natural sheet mica is obtained from muscovite or phlogopite crystals, called books. Built-up mica., made by pressing and bonding together small pieces of muscovite or phlogopite, is commonly used in place of natural sheet mica. Poor grades of muscovite and phlogopite are ground up for use as filler in gypsum plasterboard, paints, rubber, and roofing materials.
Mica can be made synthetically. The composition of synthetic mica can be varied to provide electrical and thermal insulation especially suited for specific applications.
The specific gravity of micas ranges from 2.7 to 3.3; their hardness, from 2 to 4.