greenschist

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greenschist A low-grade, regional metamorphic rock, containing abundant chlorite with albite, epidote, and sericite, and possessing a distinct cleavage. Greenschists form by metamorphism of basic igneous rocks, the chlorite component being derived from the ferromagnesium minerals present in the original rock. Chlorite imparts a green colour to the cleaved rock, hence the name ‘greenschist’.