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 |
Phyllite |
Schist. Kyrgyzstan |
schist photomicrograph |
schist outcrop, Idaho |
folded gneiss |
banding in gneiss |
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