Rock Topics

Classification of Metamorphic Rocks (Part 4)


• 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.

Metamorphic Grade

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.

SlateSlate (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.

UnakiteUnakite (Photograph by Parvinder Sethi)