Blueschist, also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15–30 km (9.3–18.6 mi). The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite.
Blueschist on Île de Groix, France
Photomicrograph of a thin section of blueschist facies metamorphosed basalt, from Sivrihisar, Turkey
Photomicrograph of garnet-lawsonite-glaucophane blueschist from Sivrihisar, Turkey (Field of view is 3 mm)
Photomicrograph of blueschist facies quartz sediment, Sivrihisar, Turkey
Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.
A cross-polarized thin section image of a garnet-mica-schist from Salangen, Norway showing the strong strain fabric of schists. The black crystal is garnet, the pink-orange-yellow colored strands are muscovite mica, and the brown crystals are biotite mica. The grey and white crystals are quartz and (limited) feldspar.
Amphibolite formed by metamorphism of basalt showing coarse texture
A mylonite (through a petrographic microscope)
A metamorphic rock, deformed during the Variscan orogeny, at Vall de Cardós, Lérida, Spain
