Blueschist with glaucophane

Classic blueschists. The slight blue tinge results from the mineral glaucophane (an amphibole), which here forms the rather stubby needles. This rock started life as a volcanic rock of basic composition, part of the old ocean floor of Tethys. Blueschists are comonly thought to be diagnostic of former subduction zones, because they imply relatively high pressure conditions relative to the temperature (compared to normal geothermal gradients).

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