Metamorphic Rocks
Most metamorphic rocks are classified according to grain size, with the coarsest grain sizes reflecting the highest temperatures (or grade) of metamorphism.  The lowest grade metamorphic rock type is slate, which consists of crystals that are generally too fine to see.  The next higher grade rock, phyllite, contains grains that are still too small to see easily, but are coarse enough to catch light and so impart a sheen to the rock.  Schist, the next higher grade rock, typically contains easily visible crystals of muscovite or biotite.  Gneiss, the highest grade metamorphic rock, contains bands of easily visible quartz, feldspar, and/or mica.
From left to right: slate, phyllite, schist, gneiss

More metamorphic rocks.  Click on each photo for a larger image.

slate in quarry

slate outcrop, Idaho


Schist. Kyrgyzstan

schist photomicrograph

schist outcrop, Idaho

folded gneiss

banding in gneiss

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