schisten

Schist is a foliated metamorphic rock formed by the medium- to high-grade metamorphism of clay-rich precursors such as shale or mudstone. It is characterized by schistosity, a planar fabric created by the preferred orientation of platy minerals, most often micas (such as muscovite and biotite). The grains are typically large enough to be seen with the naked eye, giving the rock a glittering, scaly appearance.

Mineral composition and textures vary, but schist commonly contains abundant micas along with quartz and feldspar. Other minerals, such as garnet, chlorite, tourmaline, and amphibole, may occur depending on the metamorphic conditions. The presence and arrangement of these minerals produce the characteristic aligned, flaky textures that classify the rock as schist. Several varieties are named for their dominant minerals, including mica schist, garnet-mica schist, chlorite schist, and graphite schist or talc schist.

Formation and classification: schist forms under regional metamorphism at moderate to high temperatures and pressures, typically along convergent plate margins in mountain-building belts. It represents a higher metamorphic grade than slate and phyllite, but lower than gneiss. The transformation processes involve recrystallization and the growth of platy minerals, which align into the schistosity grain-by-grain.

Occurrence and use: schist is widespread in ancient cratons and orogenic belts around the world. It is commonly used as a decorative or building stone when aesthetically appealing, and certain varieties (such as graphite or talc schist) have specialized industrial or commercial uses. Schist is distinct from non-foliated rocks like quartzite and from lower-grade foliated rocks such as slate and phyllite.