Definitions

gneiss

gneiss

[nahys]
gneiss, coarse-grained, imperfectly foliated, or layered, metamorphic rock. Gneiss is characterized by alternating light and dark bands differing in mineral composition and having coarser grains than those of schist. The light bands of gneiss are generally composed of quartz and feldspar. Hornblende, biotite mica, garnet, or graphite commonly form the dark bands. Gneisses result from the metamorphism of many igneous or sedimentary rocks, and are the most common types of rocks found in Precambrian regions. Gneiss is found in New England, the Piedmont, the Adirondacks, and the Rocky Mts. Some gneisses are used as facing stone on buildings.

Medium- to coarse-grained metamorphic rock with parallel, somewhat irregular banding that has little tendency to split along planes. Gneiss is the principal rock over extensive metamorphic terrains. Orthogneiss is formed by the metamorphism of igneous rocks; paragneiss results from the metamorphism of original sedimentary rocks. Pencil gneiss contains rod-shaped individual minerals or segregations of minerals, and augen gneiss contains large lenticular mineral grains or mineral aggregates having the appearance of eyes scattered through the rock.

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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 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 Saxon mining term that seems to have meant decayed, rotten, or possibly worthless material.

Augen gneiss

Augen gneiss 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.

Etymology: from the German Augen (ˈaʊgən), meaning "eyes".

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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