MICA-SCHIST, in petrology, a rock composed essentially of mica and quartz, and possessing a foliated or schistose struc ture due to the parallel arrangement of the mica flakes. Mica schists differ from gneisses principally by the absence of conspic uous alternations of schistose and granular bands characteristic of the latter rocks, and by the paucity of felspar minerals. Between phyllite and mica-schist there are all gradations. Mica schists possess a greater variety of constituent minerals and are more coarsely crystallized. The mica may be muscovite or biotite ; both are often present. Paragonite and fuchsite are rare. In addition to quartz, minor quantities of albite or oligoclase fel spar are frequently found. A great number of accessory minerals are known in mica-schists, and when these are conspicuous they may be regarded as constituting special varieties receiving dis tinctive names. These minerals include almandine garnet, stauro lite, kyanite, andalusite and sillimanite. Many of them are indicators of a special grade of metamorphism. Minor accessories include rutile, hematite, ilmenite, tourmaline and zircon.
In nearly all cases mica-schists are metamorphosed sedimentary rocks of the composition of clays, shales and slates, though examples are known which are clearly transmuted acid igneous rocks, such as rhyolites and porphyries. The origin of these latter
types, if not decipherable by their field relations, is usually indicated by their bulk chemical composition.
The common associates of mica-schist are quartzites, quartz schists and limestones, representing sediments of a siliceous and calcareous nature interstratified with clays and mudstones from which the mica-schists are derived.
Like all metamorphic rocks mica-schists are principally found in areas of pre-Cambrian rocks. In the mountain chains of the Alps, the Himalayas, etc., mica-schists of Palaeozoic and Mesozoic age are known as they are seen to pass into sediments containing recognizable fossils. At Bergen, in Norway, Palaeozoic fossils have been found in mica-schists, and some of the Mesozoic schists of the Alps retain identifiable organic remains. Mica-schists are rarely of economic value, being too fissile for building-stones and too brittle for roofing slates. (See also ScHIsTs.) (C. E. T.)