Schist is a low- to medium-grade metamorphic rock with a well-defined foliation and containing minerals that are visible with the unaided eye. The name derives from the Greek schizo, ‘to tear’, in reference to its very strong fissility. Indeed, schists display a penetrative foliation – known as schistosity – defined by platy and elongated minerals oriented parallel to each other. Schistosity-defining minerals are, in the vast majority of cases, phyllosilicates (white mica, biotite, chlorite, talc…) and/or amphiboles. Many schists contain a very fine-grained layering with domains containing foliation-defining minerals interlayered with domains with granular phases (like quartz, feldspar, or pyroxenes). Schists may contain a wide range of silicates (e.g. garnet, chloritoid, staurolite, kyanite…) and accessory minerals (graphite, ilmenite…), depending on the chemical composition and the metamorphic grade. Any distinctive mineral present should be specified in the name of the rock (e.g. andalusite schist).
The term ‘schist’ is not restricted to coarse-grained schistose rocks but represents also a general term encompassing fine-grained varieties, like slate e phyllite. The boundary with the higher-grade rock gneiss is defined based on the spacing of foliation, which is less than 1 cm in schists and more than 1 cm in gneisses. In case of doubt, the term ‘gneiss’ should be used in preference for rocks that display well-defined compositional bands and very coarse grain size. Schists derive from the metamorphism of sedimentary rocks (mudstone, sandstone, marl…), volcanic rocks (like tuffs), and mafic igneous rocks (e.g. basalt). They are the medium-grade equivalents of phyllites and slates.