gneiss

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gneiss

(nīs), coarse-grained, imperfectly foliated, or layered, metamorphic rockrock,
aggregation of solid matter composed of one or more of the minerals forming the earth's crust. The scientific study of rocks is called petrology. Rocks are commonly divided, according to their origin, into three major classes—igneous, sedimentary, and metamorphic.
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. 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 quartzquartz,
one of the commonest of all rock-forming minerals and one of the most important constituents of the earth's crust. Chemically, it is silicon dioxide, SiO2.
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 and feldsparfeldspar
or felspar
, an abundant group of rock-forming minerals which constitute 60% of the earth's crust. Chemically the feldspars are silicates of aluminum, containing sodium, potassium, iron, calcium, or barium or combinations of these elements.
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. 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.

gneiss

A coarse-grained, dark metamorphic rock; composed mainly of quartz, feldspar, mica, and other minerals corresponding in composition to granite, in which the minerals are arranged in layers. See also: Stone

Gneiss

 

a metamorphic rock, foliated, rich in feldspar, and containing smaller quantities of such minerals as quartz, bio-tite, muscovite, amphibole, pyroxene, garnet, and sillimanite.

The formation of gneiss is associated with the deep metamorphism of various rocks and sometimes with fusion because of easily fused granite material in these rocks. As a result of the flow of material, certain granites can acquire the features of gneiss. Gneisses are subdivided into orthogneisses and paragneisses. Orthogneisses are formed from igneous rocks and paragneisses from sedimentary rocks. The foliation of gneiss also varies. In some cases it is a remnant of the initial layering of sedimentary rocks, while in other cases it is a remnant of layered intrusions of igneous material. In addition, foliation may be the result of the viscous flow of fused material during metamorphism. Gneiss is very widely distributed, especially in Precambrian rocks. In folded regions there are gneisses that have been formed by changes in younger rocks under the influence of the pressure developed during the folding process. Gneiss is used to make gravel, pavement slabs, and facing materials.

REFERENCE

Kuznetsov, E. A. Kratkii kurs petrografii (magmaticheskikh i metamorficheskikh porod). Moscow, 1970.

gneiss

[nīs]
(petrology)
A variety of rocks with a banded or coarsely foliated structure formed by regional metamorphism.

gneiss

A coarse-grained metamorphic rock having discontinuous foliation; usually dark; composed mainly of quartz, feldspar, mica, and ferromagnesian minerals. Generally classed as trade granite in the building stone industry.

gneiss

any coarse-grained metamorphic rock that is banded and foliated: represents the last stage in the metamorphism of rocks before melting