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 . The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite. Blueschists are schists typically found within orogenic belts as terranes of lithology in faulted contact with greenschist or rarely eclogite facies rocks. Petrology Blueschist, as a rock type, is defined by the presence of the minerals glaucophane + ( lawsonite or epidote ) +/- jadeite +/- albite or chlorite +/- garnet +/- muscovite in a rock of roughly basaltic composition. Blueschist often has a lepidoblastic, nematoblastic or schistose rock microstructure defined primarily by chlorite, phengitic white mica, glaucophane, and other minerals with an elongate or platy shape. Grain size is rarely coarse, as mineral growth is retarded by the swiftness of the rock's metamorphic trajectory and perhaps more importantly, the low temperatures of metamorphism and in many cases the anhydrous state of the basalts. However, porphyritic varieties do occur. Blueschists may appear blue, black, gray, or blue-green in outcrop. Blueschist facies Blueschist facies is determined by the particular temperature and pressure conditions required to metamorphose basalt to form blueschist. Felsic rocks and pelitic sediments which are subjected to blueschist facies conditions will form different mineral assemblages than metamorphosed basalt. Thereby, these rocks do not appear blue overall in color. Blueschist mineralogy varies by rock composition, but the classic equilibrium assemblages of blueschist facies are: Basalts: glaucophane + lawsonite and/or epidote + albite + titanite +/- garnet +/- quartz jadeite + quartz - diagnostic of pressures ~> 10 kbar Ultramafic rocks: serpentinite/lizardite +/- talc +/- zoisite Pelites: Fe-Mg-carpholite +/- chloritoid +/- kyanite + zoisite +/- pargasite or phengite +/- albite +/- quartz +/- talc +/- garnet Granites: kyanite +/- paragonite +/- chlorite +/- albite +/- quartz +/- pargasite or phengite Calc-silicates: Various Limestones and marble: calcite transforms to aragonite at high pressure, but typically reverts to calcite when exhumed Blueschist facies generally is considered to form under pressures of >0.6 GPa, equivalent to depth of burial in excess of 15–18 km, and at temperatures of between 200 and 500 °C. This is a 'low temperature, high pressure' prograde metamorphic path and is also known as the Franciscan facies series, after the west coast of the United States where