Blueschist

Blueschists are schists typically found within orogenic belts as terranes of lithology in faulted contact with greenschist or rarely eclogite facies rocks.

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.

Felsic rocks and pelitic sediments which are subjected to blueschist facies conditions will form different mineral assemblages than metamorphosed basalt.

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 these rocks are exposed.

His carefully constructed definition established the pressure and temperature conditions which produce this type of metamorphism.