gneiss

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Related to Gneisses: Cambrian period, orthogneiss

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
References in periodicals archive ?
The plan of the geological structure (Huhma, 1975) shows that the wellhead is located in the field of Proterozoic mica gneisses (schists).
The complex consists of pyroxene gneisses, quartz-feldspar gneisses, biotite-plagioclase gneisses, amphibole gneisses, garnet-cordierite gneisses (Vaivara complex), and magnetite-rich quartzites and gneisses (Johvi magnetic anomaly).
These authors interpret isotopic variations in galena as reflecting a general uniformity of Th/Pb and variability of U/Pb in the Grenville gneisses.
This study of the dykes is being carried out in conjunction with our detailed studies of the field relationships, structure, petrology, and geochemistry of the diverse components of the gneisses and greenstone belt at Isua.
A dominant planar structural element of rock mass of the deposits Rozna and Olsi is the metamorphous foliation, which is defined, on an approximate scale, by interlaminating of individual lithological units with the layers of gneisses, amphibolites, the intercalated beds of erlans, and crystallic limestones with layers abounding in sulphides.
On the NE end of the profile, the low-density upper crust consisting of mica schist and mica gneisses, juxtaposed to the thick (~30 km) lower crust, produces a great mass deficiency.
Quartzofeldspathic veins and sulfide mineralization zones within the Chakdara granite gneisses have economically low concentration of Au, Ag and base metals.
On the assumption that rocks from surface are analogues of tested gneisses from the KSDB-3 (the same modal composition and grain-size characteristics), the differences in the other physical and deformation properties between core samples and analogues arose due to P-T conditions in the deep borehole.
In South Estonia, the mineral parageneses of intermediate and mafic metavolcanics and Al-rich gneisses correspond to the granulite facies.
To date geological mapping has documented numerous areas of artisanal bedrock mining operations hosted within shallow dipping garnetiferrous, quartzo-feldspathic and biotitic gneisses, scattered over a surface area of 14 square kilometres in the east-central portion of the Morogoro property.
55% metamorphic rocks including schists, gneisses, marbles, slates and epidosite and 34.