gneiss

[German Gneis, probably alteration of Middle High German ganeist, spark (from its appearance), from Old High German gneista.]

Coarse-grained, banded crystalline rock. Gneiss is composed of mineral grains large enough to be seen with the naked eye (see illustration). Banding arises from segregation of the various minerals present, typically into dark- and light-colored layers. Individual bands are commonly 0.04 to 0.4 in. (1 mm to 1 cm) thick. Although individual mineral grains are often flattened parallel to banding, such shape orientation is not present in many gneisses. Sheetlike minerals such as micas may be present but form only a subordinate amount of the rock. Banded rock of coarse grain containing substantial amounts of such minerals is named schist. Crystalline rock which has flattened grains but lacks obvious banding is generally called leptite. See also Schist.

Gneiss formed by metamorphism of preexisting granite. Dark minerals are mica; light-colored minerals are quartz and feldspar. The streaky nature of banding is typical of gneisses. The sample is from the Great Smoky Mountains of North Carolina.

Gneiss is defined by its texture, or arrangement of mineral grains, rather than by its mineral composition. However, the term gneiss is often taken to imply a mineral composition of granitic type, dominated by quartz and feldspar. Gneisses of other compositions are identified by qualifying terms such as compositional rock names, as in diorite gneiss and amphibolite gneiss, or a partial list of minerals present, as in biotite-plagioclase gneiss and hornblende-plagioclase gneiss. See also Feldspar; Quartz.

Most gneisses are formed by recrystallization of preexisting rock during intense regional metamorphism. Shear stress present during such metamorphism causes formation of gneissic banding, although the exact mechanisms of this process are not well understood. Gneisses typically occupy large areas within the high-grade cores of regional metamorphic belts. Such terranes are often difficult to understand, because the processes which cause formation of gneissic texture are also sufficient to obscure preexisting rock structures. High temperature and shear are sufficient to cause plastic flow of gneissic rock on a gigantic scale. Such conditions of metamorphism are probably brought about by deep tectonic burial and major regional compression. Thus gneissic terranes may be expected to form in areas of convergent plate tectonics. See also Metamorphic rocks; Metamorphism; Metasomatism; Plate tectonics.

A highly metamorphosed rock of a granular texture and with a banded appearance. Gneisses may have been metamorphosed from schists or quartzites, themselves metamorphic rocks. Some gneisses may be produced by the interaction of igneous magma with metamorphic rocks. Gneisses are resistant rocks, but inclined to exfoliation along the sheet joints.

For more information on gneiss, visit Britannica.com.

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.

Tutor's tip: My "niece" (my brother's or sister's daughter) can gather some "nice" (pleasant) to look at "gneiss" (a type of rock).

Gneiss rock.

Study of Gneiss Rock, Glenfinlas, the Trossachs, Scotland. A pen and ink study by John Ruskin, 1853, now in the Ashmolean Museum, Oxford.

Gneiss (pronounced /ˈnaɪs/)(also known as "bastard granite" or "stratified granite") is a 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.^[1] Other sources claim the root to be an old Saxon mining term that seems to have meant decayed, rotten, or possibly worthless material.^{[citation needed]}

Contents 1 Augen gneiss 2 See also 3 References 4 Footnotes

Augen gneiss

Augen gneiss from Rio de Janeiro, Brazil

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 (IPA: [ˈaʊɡən]), meaning "eyes".

See also

• List of rock types

References

Wikimedia Commons has media related to: Gneiss

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

Footnotes

1. ^ Online Etymology Dictionary

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)