Greenschist

A metamorphic rock that develops under the lowest temperatures and pressures typically produced by regional metamorphism, around 300 to 450 degrees Celsius and 2 to 10 kilobars, usually from mafic volcanic rocks such as basalt and gabbro. It owes its green color and name to the minerals chlorite, epidote and actinolite, and it also commonly contains platy minerals such as muscovite along with quartz, orthoclase, talc, carbonate minerals and the amphibole actinolite. The alignment of its platy minerals during metamorphism produces a pronounced foliation called schistosity, splitting the rock into thin layers generally less than 5 to 10 millimeters thick, and its defining mineral assemblage for a basaltic parent rock is chlorite, actinolite and albite, with or without epidote. The greenschist facies as a whole spans roughly 400 to 500 degrees Celsius at depths of about 8 to 50 kilometers, a characteristic low temperature, moderate pressure stage along ordinary geothermal gradients.