Mica (Lat. Micare, to sparkle), in mineralogy, the name of a group of the silicates, distinguished by their remarkable lamellar structure, the elasticity of their laminos, and their half metallic lustre. The minerals crystallize in right rhomboidal prisms of 120°, which separate with the greatest facility in folire parallel with the base of the crystal. These may be subdivided till many thousand plates are required to make the thickness of an inch. They are found usually transparent, elastic, and tough. The colors are various; the most common are silvery white, grayish green, red, and black. The hardness of the mineral is 2 to 3; specific gravity 2.65 to 3.3. The different species are distinguished partly by their different optical characters as well as by their differences of composition. They present two axes of double refraction, which, in the species designated by Dana as muscovite, and commonly known as Muscovy glass, vary in apparent inclination between 44° and 75°; in the phlo-gopites, called also rhombic mica and magnesia mica in part, from 5° to 20°; and in the biotites below 5°. Prof. B. Silliman, jr., observes that the muscovites are confined to granitic and other igneous rocks, and the phlo-gopites to granular limestone and serpentine.

The former generally contain potash or lithia and little magnesia, and the latter contain magnesia, and often but little alkali. The composition of the most common micas, according to Dufrenoy, is from 45 to 50 per cent, of silica, 32 to 33 of alumina, 10 to 15 of alkali (rarely soda), and 2 to 4 of fluoric acid. He considers the differences of composition too great to admit of any general formula. The micas are unisilicates, containing, besides silica and fluorine, alumina, iron, magnesia, potash, lithia, rubidia, and cassia, the magnesia generally failing in the varieties found in the granitic rocks. Lepidolite is a species distinguished for its occurrence usually in granular masses made up of foliated scales of rose-red color, violet gray, yellowish, or whitish. Muscovite, the most familiar form of mica, is a constituent of granite, gneiss, mica slate, and some other kindred rocks. It is found both disseminated and in veins, and in many of the stratified rocks it is an incidental constituent derived from the destruction of the original formations to which it belonged. The mineral is thus seen to be very generally distributed; but certain localities are distinguished for the production of large plates of it.

In Siberia they have been found more than 3 ft. across, and they have been obtained of great size in Sweden and Norway. This is also the case at Acworth, Grafton, and Alstead, N. H.; and mica has been found in some of the other states and in Canada sufficiently large to be quarried for economical purposes. Mica is used mostly for the doors of stoves and the sides of lanterns, for which it is well adapted by its transparency and refractory character. It has been used as a substitute for window glass, and its toughness recommends it for this purpose on board vessels of war, in which the concussion from the discharge of heavy guns might occasion the fracturing of glass. It has also been used for spectacles, optical instruments, and bronzing powder, and is serviceable for holding small objects for microscopic examination. Very extensive mica mines were discovered in Mitchell co., N. C, in 1867, which have since been extensively worked. They had evidently been worked centuries ago.