Classification of Metamorphic Rocks (Part 4)
Composition
• Common minerals in metamorphic rocks include quartz, feldspar, mica, calcite, and hornblende.
• Index minerals in metamorphic rocks indicate how much the original rock was metamorphosed, called the grade of metamorphism.
Index Minerals minerals include chlorite, epidote, garnet, staurolite, kyanite and sillimanite.
Fine-grained crystals of chlorite (not visible) give this slate its green color. Chlorite is a complex silicate mineral formed during low-grade metamorphism.
Slate (Photograph by Parvinder Sethi)
Green crystals of epidote are visible in this sample of unakite. Epidote is a complex silicate mineral formed during low- to medium-grade metamorphism.
Unakite (Photograph by Parvinder Sethi)