schist, metamorphic rock having a foliated, or plated, structure called schistosity in which the component flaky minerals are visible to the naked eye. Schists are distinguished from the other foliated rocks, slates and gneisses, by the size of their mineral crystals; these are larger than those of slates, being visible to the naked eye, but smaller than those of gneisses, which are coarsely foliated rocks as opposed to finely foliated, or schistose, rocks. As contrasted with the folia of slates, the folia of schists are rough-surfaced and irregular. Schists split readily along their planes of schistosity, like slates along cleavage lines. Like other foliated rocks, schists owe their origin to the metamorphism of preexisting rocks. The commonest of the schists is mica schist, the essential minerals of which are quartz and mica (biotite or muscovite). Other schists are hornblende schist, talc schist, chlorite schist, and graphite schists. Schists are abundant in the Precambrian (Archean and Proterozoic) rocks.

Crystalline metamorphic rock that has a highly developed tendency to split into layers. Most schists are composed largely of platy minerals such as muscovite, chlorite, talc, biotite, and graphite. The green colour of many schists and their formation under a certain range of temperature and pressure conditions have led to distinction of the greenschist facies in the mineral facies classification of metamorphic rocks. Schists are usually classified on the basis of their mineralogy, with varietal names that indicate the characteristic mineral present.

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One of the major divisions of the mineral facies classification of metamorphic rocks, encompassing rocks whose peculiar mineralogy suggests that they formed under conditions of high pressure and relatively low temperature (generally less than about 662°F, or 350°C); such conditions are not typical of the normal geothermal gradient in the earth. The minerals that chiefly occur include soda amphibole (glaucophane), soda pyroxene (jadeite), garnet, lawsonite, and pumpellyite. Quartz, muscovite, chlorite, epidote, and plagioclase may also be present. Classic deposits occur in western California.

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