Gneiss

The gneisses are common and widely distributed rocks which have been derived by metamorphic processes from preexisting formations that were originally either igneous or sedimentary rocks. Gneissic rocks are coarsely laminated and largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. However, depending on their composition, they may also be called garnet gneiss, biotite gneiss, albite gneiss, etc. Orthogneiss designates a gneiss derived from an igneous rock and paragneiss one from a sedimentary rock.

The word "gneiss" is from an old Saxon mining term which seems to have meant decayed or rotten, or possibly worthless material.

See also: List of rocks


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