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Greenschist

Pictures of Metamorphic Rock Types

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Greenschist forms by regional metamorphism under conditions of high pressure and fairly low temperature. It isn't always green or even a schist. (more below)
A facies more than a rock type
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
Greenschist is the name of a metamorphic facies, a set of typical minerals that form under specific conditions, in this case relatively cool temperatures at high pressures that are less than those of blueschist. Chlorite, epidote, actinolite and serpentine are the green minerals that give this facies its name, but whether they appear in any given greenschist-facies rock depends on what the rock originally was. This greenschist specimen is from northern California, where seafloor sediment has been subducted beneath the North American plate, then thrusted to the surface soon afterward as tectonic conditions changed.

This specimen consists mostly of actinolite. The vaguely defined veins running vertically in this image may reflect the original bedding in the rocks from which it formed. These veins contain mainly biotite.

For more photos see the Metamorphic Rocks Gallery.

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