schist

Schist is a medium- to high-grade metamorphic rock characterized by well-developed schistosity, a planar foliation produced by the alignment of platy minerals such as micas (primarily muscovite and biotite) and chlorite. The constituent grains are typically large enough to be seen with the naked eye, giving the rock a glittery or shiny appearance.

Formation and textures: Schist forms mainly through regional metamorphism of pelitic rocks such as mudstones and shales under elevated temperature and pressure during tectonic plate convergence. This causes minerals to recrystallize and align in parallel layers, producing the characteristic foliation. Porphyroblasts of garnet, staurolite, kyanite, or sillimanite may occur, indicating higher metamorphic grade.

Subtypes: Common varieties include mica schists, which are dominated by muscovite and/or biotite; chlorite schist, which is greener and lower-grade; and garnet schist, which contains conspicuous garnet crystals. Schist differs from slate and phyllite by having coarser grain size and a more pronounced foliation, and it differs from gneiss by lacking the strong compositional banding typical of gneiss.

Occurrence and significance: Schist is widespread in mountain belts around the world and in zones of regional metamorphism. It often forms from pelitic protoliths but can derive from other sedimentary or igneous materials subjected to metamorphism. Economically, schist can host various mineral deposits and, when well foliated and aesthetically suitable, may be used as decorative stone or dimensional stone in some contexts. The term derives from the German schiefer, meaning slate.