Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphism of pre-existing formations that were originally comprised of either igneous, sedimentary, or metamorphic rocks. Gneissic rocks are usually medium to coarse grained, foliated, and largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses whose protoliths were igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, and the like. However, depending on their composition, they may also be called garnet gneiss, biotite gneiss, albite gneiss, based upon composition only and not upon the original protolith class. The term orthogneiss is applied to a gneiss derived from an igneous rock, and the term paragneiss is one from a sedimentary rock. Gneissose is a used to describe rocks with textural properties similar to gneiss. Gneiss resembles schist, except that the minerals are arranged into bands and there is a paucity of platy minerals such as mica. It is often difficult to tell the difference between gneiss and a schist as they are often found in close association, with a gradational contact relationship.
