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.