Gneiss (redirected from Lewisian gneiss)

gneiss (nīs), 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.

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gneiss

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

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

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gneiss [nīs]

(petrology)

A variety of rocks with a banded or coarsely foliated structure formed by regional metamorphism.

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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.

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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.