Mica (Lat. micare, 'to glitter'), the name given to an important group of rock-forming minerals, which have all a hardness of about 2, and are characterised by their perfect cleavage in one direction—the laminæ being flexible and elastic. They are essentially aluminous silicates containing potash, or soda, or lithia, and in some species magnesia along with potash. They all crystallise in monoclinic forms which approximate closely to hexagonal and rhombic crystals. Muscovite or potash mica is a silicate of alumina and potash, with some of the latter occasionally replaced by soda and small quantities of magnesia, ferrous oxide, and fluorine. It is seldom colourless, but usually yellowish, brownish, or greenish. The lustre is pearly or almost metallic. The thin plates into which it divides are generally transparent, and were formerly much used in setting objects for the microscope, but for this purpose thin glass is now preferred. It is still useful for the mounts of natural history objects which are to be put in spirit, being more easily bored than glass. Plates of muscovite often a yard across are found near Lake Baikal, at Aeworth in New Hampshire, and in China. Large plates also occur in Sweden and in Norway, and masses of the mineral are met with in Cornwall. In Siberia, China, Peru, and elsewhere it is used as a substitute for glass in windows. It is sometimes preferred to glass for lanterns, and especially for the fronts of stoves, as not being liable to break with sudden changes of temperature. In India small pictures are frequently painted in distemper on mica. Muscovite occurs as one of the essential constituents of ordinary granite, gneiss, and mica-schist. It is also an ingredient of many other plutonic rocks and crystalline schists, but is not a primary constituent of volcanic rocks: where present in the latter it is as an alteration-product. Sericite is a talc-like variety of muscovite, not uncommonly met with as a constituent of certain schistose rocks, to which it imparts a silky lustre on the planes of foliation. Damourite, somewhat like sericite, is also a variety of muscovite which occurs occasionally in schistose rocks. Lepidolite or lithia mica is a silicate of alumina with potash and lithia; white, rose-red, or violet as a rule, but sometimes greenish; does not occur in measurable crystals but in irregular plates and tables, and now and again in scaly, granular, or compact aggregates. In Moravia a massive granular lepidolite is found with a fine reddish-violet colour. Like jasper, lapis-lazuli, &c., lepidolite is made into ornaments; as a rock-forming mineral it is of small account. Another lithia mica containing iron is called Zinnwaldite. Biotite or magnesia mica embraces several varieties which in addition to potash contain magnesia, the latter being generally replaced in part by ferrous oxide. When ferrous oxide entirely replaces magnesia we have iron mica. The most important of the biotites is Meroxene, which occurs in fine crystals on Vesuvius and in other volcanic regions. It was from a study of the fine specimens of meroxene occurring in ejected blocks at Vesuvius that mineralogists were able to determine the crystalline form of mica. Formerly the micas were assigned either to the hexagonal or to the orthorhombic system. Meroxene is met with also in such rocks as granite, gneiss, &c., in scaly granules and plates, often associated with muscovite, and sometimes forming scaly aggregates. It is dark green or brown, sometimes yellowish, but generally very dark or even black, hence it is often called black mica. In volcanic rocks (basalt, trachyte, &c.) it occurs sparingly in the form of small scales or plates. Altered forms of meroxene are the red Rubellane, occurring in many volcanic rocks, and the black Voigtite, met with in granitic rocks. Phlogopite is a biotite of a reddish-brown, but sometimes yellow or even greenish colour. It contains a larger proportion of silica than meroxene. Anomite is another biotite only to be distinguished from meroxene by some optical characters. Lepidomelane is a magnesia mica rich in ferrous and ferric oxides; many of the magnesia micas occurring in granite, gneiss, &c. belong to this variety. The biotites are much more readily decomposed than the muscovites, being often altered into chloritic minerals with epidote and calcite. Paragonite or soda mica, an aluminous silicate of soda, occurs chiefly in certain crystalline schists, and is known only in the form of small white or colourless scales. It closely resembles muscovite, from which it can only be distinguished chemically.
MICA-SCHIST is, next to gneiss, one of the most abundant of the crystalline schists. It consists of alternate layers of mica and quartz, but is sometimes composed almost entirely of the thin and shining plates or scales of mica, and from this it passes by insensible gradations into phyllite, as this in turn passes into clay-slate. The quartz occurs pure in thin layers like vein-quartz, thinning off and swelling out abruptly. Sometimes it appears as irregular swollen-shaped lumps round which the folia of mica are arranged. The mica is usually muscovite, but occasionally it is biotite. Many accessory minerals are found in mica-schist, especially garnets: others are schorl, kyanite, hornblende, andalusite, beryl, &c. In many places the mica-schist has a finely corrugated or wavy structure.