schist (shĭst) [key], 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.
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