Gneiss
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Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from preexisting formations that were originally either igneous or sedimentary rocks. Gneissic rocks are coarsely foliated 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 is one from a sedimentary rock. Gneissose is used to describe rocks with properties similar to gneiss.
The word "gneiss" is from an old Saxon mining term that seems to have meant decayed, rotten, or possibly worthless material.
The Acasta Gneiss is the oldest known crustal rock in the world.
Augen gneiss
Augen gneiss from Rio de Janeiro, Brazil
A coarse-grained gneiss, probably originated from metamorphism of igneous granite, whith characteristics elliptic or lenticular feldspaths, normally microcline, contrasting with the ribbon shape of the quartz, biotite and magnetite clusters.
Etymology: from the German augen, meaning "eyes".
See also
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