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Blueschist

Pictures of Metamorphic Rock Types

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Blueschist signifies regional metamorphism at relatively high pressures and low temperatures, but it isn't always blue, or even a schist. (more below)
Not always a blue schist
Photo (c) 2005 Andrew Alden, licensed to About.com (fair use policy)
High-pressure, low-temperature conditions are most typical of subduction, where marine crust and sediments are carried beneath a continental plate and kneaded by changing tectonic motions while sodium-rich fluids marinate the rocks. Blueschist is a schist because all traces of original structure in the rock have been wiped out along with the original minerals, and strongly layered fabric has been imposed. The bluest, most schistose blueschist—like this example— is made from sodium-rich mafic rocks like basalt and gabbro.

Petrologists often prefer to talk about the glaucophane-schist metamorphic facies rather than blueschist, because not all blueschist is all that blue. In this hand specimen, from Ward Creek, California, glaucophane is the major blue mineral species, but in other examples lawsonite, jadeite, epidote, phengite, garnet, and quartz are also common. It depends on the original rock that is metamorphosed. For instance, a blueschist-facies ultramafic rock consists mainly of serpentine (antigorite), olivine and magnetite.

As a landscaping stone, blueschist is responsible for some striking, even garish effects.

For more photos see the Metamorphic Rocks Gallery.

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