DEFINITION OF GREENSCHIST

Metamorphic rocks are commonly referred to in

the field by informal names that reflect some distinctive physical

or mineralogical aspect of the rock. A greenschist is an

informal name for a schistose, generally dark green rock (on

fresh surfaces), whose color is due to the presence of the metamorphic

minerals chlorite, actinolite, epidote, or serpentine. In

more formal usage, a greenschist is a schistose rock that has

been metamorphosed to the greenschist facies, typically in the

range of 570°F–930°F (300°C–500°C), at low to medium pressures.

It is a typical low-grade regional metamorphic rock. The

greenschist facies is defined in metamorphosed basic (mafic)

rocks by the mineral assemblage albite + epidote + chlorite +

actinolite. In metamorphosed pelitic rocks, the greenschist

facies assemblage typically includes chlorite + white mica +

biotite + chloritoid.

See also GREENSTONE BELTS; METAMORPHISM.

greenstone See GREENSTONE BELTS.

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