Gneiss
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[[Image:Gneiss.jpg|thumb|Gneiss]]
'''Gneiss''' ({{pronEng|ˈnaɪs}}) is a common and widely distributed type of [[Rock (geology)|rock]] formed by high-grade regional [[metamorphic rock|metamorphic]] processes from preexisting formations that were originally either [[igneous rock|igneous]] or [[Sedimentary rock|sedimentary]] rocks. Gneissic rocks are usually medium to coarse [[Foliation (geology)|foliated]] and largely [[Recrystallization|recrystallized]] but do not carry large quantities of [[mica]]s, [[Chlorite group|chlorite]] or other platy [[mineral]]s. 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 language|English]] at least since 1757<ref>[http://www.etymonline.com/index.php?search=gneiss&searchmode=none Online Etymology Dictionary]</ref>. Other sources claim the root to be an old [[Saxony|Saxon]] mining term that seems to have meant decayed, rotten, or possibly worthless material.
== See also ==
* [[List of rock types]]
==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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