Definition (Encyclopedia of Global Resources)
In the narrowest meaning of “gneiss” (pronounced “nice”), it is defined as a coarse-grained, feldspar-rich, metamorphic rock with a parallel structure (foliation) that assumes the form of streaks and bands.
Gneiss is primarily identified by its structure rather than by its composition. It is a medium- to coarse-grained banded or coarsely foliated crystalline rock. The rock is characterized by a preferred orientation of platy grains such as biotite, muscovite, or hornblende, or the segregation of minerals into bands or stripes. Unlike schist, gneiss is more often characterized by granular minerals than by platy minerals. Most gneisses are light to dark gray, pink, or red because of the high feldspar content.
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Overview (Encyclopedia of Global Resources)
Gneiss is exposed in regions of uplift where erosion has stripped away surficial rocks (sediments and lower grade metamorphic rocks) to expose rocks that have been altered at depth. In North America, gneiss may be found in New England, in the central Atlantic states, the Rockies, the Cascades, and much of Canada.
Gneiss, with mineralogy similar to that of granite, has similar uses except that it is generally restricted by the presence of a higher percentage of ferromagnesium minerals and micas, which weather rapidly to weaken and discolor the finished stone. The major use is as riprap, aggregate, and dimension stone. Wavy foliation in polished slabs results in an especially decorative stone for monuments.
The most common gneisses are similar to granite in composition and resemble granite except for the foliation. The predominant minerals are equidimensional grains of quartz and potassium feldspar, usually microcline. Sodium plagioclase may also be present. Biotite, muscovite, and hornblende, alone or in combination, are the most common minerals that define the foliation. Other minerals, almost exclusively metamorphic in origin, that may be present in minor quantities include almandine garnet, andalusite, staurolite, and sillimanite.
True gneiss is a high-grade metamorphic rock formed by recrystallization and chemical reaction within existing rocks in response to high temperature and pressure at great depths in the...
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Gneiss (World of Earth Science)
Gneiss (pronounced "nice") is a metamorphic rock consisting mostly of quartz and feldspar and showing distinct layering or banding. The layering of a gneiss may be weak or well-developed and consists of varying concentrations of biotite, garnet, hornblende, mica, and other minerals. These structures do not record a layered deposition process but arise from preferential recrystallization along flow or stress lines during metamorphosis of the parent rock (protolith).
The gneisses are a very varied group, including both igneous rocks and metamorphosed sedimentary rocks, and may be categorized as quartzofeldspathic, pelitic, calcarous, or hornblende gneiss.
Quartzofeldspathic gneiss forms by metamorphosis of either silicic igneous rocks such as granite, rhyolite, and rhyolitic tuffr silicic sedimentary rocks such as sandstone. Quartzofeldspathic gneiss containing eye-shaped feldspar crystals is termed augen gneiss after the German augen (eyes).
Pelitic gneiss is formed by metamorphosis of clay-textured sedimentary rocks, particularly those rich in iron.
Calcareous gneiss contains calcite (CaCO3). It is formed by metamorphosis of limestones and dolomites containing large fractions of sand and clay. Calcareous gneisses with large fractions of calcite blur conceptually with the marbles (metamorphosed limestones).
Hornblende gneiss contains a large fraction of hornblende in addition to its quartz and feldspar.
The gneisses can be alternatively categorized simply as orthogneisses and paragneisses. The former are metamorphosed from igneous protoliths and the latter from sedimentary protoliths.
The gneisses and schists are closely related. Both are metamorphosed igneous or sedimentary rocks showing foliation or layering. The difference is primarily one of degree; schists are less coarsely crystallized and more prone to cleave into flakes or slabs. Gneisses represent a higher grade of metamorphosisore thorough meltingnd are distinguished by their coarser texture and their resistance to cleavage.
See also Migmatite