Mica is a generic name for a group of complex hydrous potassium-aluminum silicate minerals that differ somewhat in chemical composition; examples are biotite, lepidolite, muscovite, phlogopite, and vermiculite. Mica has a low coefficient of expansion, high dielectric strength, good electrical resistivity, a uniform dielectric constant, and capacitance stability; at one time it was the best electrical and thermal insulator known. The iron content determines the color. Muscovite is generally gray, green, or brown; biotite, brown or black; lepidolite, pink or green; phlogopite, light brown to yellow; and vermiculite, brown. Muscovite has the greatest commercial value and is the mica that is ground and pulverized into pigment grades. Muscovite crystals develop in booklike form with a well-developed basal cleavage that allows splitting the large books into extremely thin sheets or grinding the flakes into thin leaves to produce dry ground mica. These leaves have a diameter-to-thickness ratio of more than 25 to 1--a ratio higher than that of any other flaky mineral.

Large crystals or books of mica--ranging from less than 2 cm (0.8 in) up to 2 m (6.6 ft) in length--are generally found in granitic pegmatites, which are light-colored, coarsely crystalline, igneous rocks. Variation of size within an individual deposit is not uncommon. Deposits of mineral materials containing some form of mica exist throughout the world. The largest resources of muscovite are in Brazil, Western Africa, and the Madras and Bihar areas of India. The Malagasy Republic is the major world source of phlogopite mica. Mica was first mined in the United States in New Hampshire. After about 1870 production of mica began on a large scale in North Carolina, which now produces more dry and wet ground mica than does any other state, and the United States is the dominant world source of muscovite scrap and flake micas. Small dry ground mica flakes are used as a thin coating on rubber surfaces to overcome tackiness and sticking. In exterior house paints dry ground mica adds body, reduces running and sagging, and improves weatherability. The addition of mica to all types of sealers for porous surfaces (such as wallboard, masonry, and concrete blocks) greatly reduces penetration and improves holdout. The inclusion of mica in road and highway paints improves wearability, gives good adhesion, and reduces flaking and cracking. Micas are also used in caulking compounds,lubricants,greases,welding rodcoatings, and dry-powder fire extinguishers. Wet ground mica is produced by grinding mica flakes in water until they are reduced to fine scales. Wet ground mica costs more than twice as much as dry ground mica and is used predominantly in paint and rubber, as well as in plastics and lubricants. Wet ground mica is also used to coat wallpaper, because it imparts an attractive silky or pearly luster.