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Gneiss ( ) is common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. Gneissic rocks are usually medium to coarse 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.

Gneiss resembles schist, except that the minerals are arranged into bands. Sometimes it is difficult to tell the difference between gneiss and a schist because some gneiss appears to have more mica than it really does. This is especially true with mica-rich parting planes.

The etymology of the word "gneiss" is disputed. Some sources say it comes from the Middle High German verb gneist (to spark; so called because the rock glitters) and has occurred in English at least since 1757. Other sources claim the root to be an old Saxonmarker mining term that seems to have meant decayed, rotten, or possibly worthless material.

Augen gneiss

Augen gneiss, from the German Augen ( ), meaning "eyes", is a coarse-grained gneiss, interpreted as resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear bound feldspar porphyroclasts, normally microcline, within the layering of the quartz, biotite and magnetite bands.

See also



References

  • Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., pp. 359–365, Freeman, ISBN 0-7167-2438-3


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