Gneiss

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Gneiss

Gneiss is a common and widely distributed type of rock

Rock (geology)

In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock....
formed by high-grade regional metamorphic

Metamorphic rock

Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form"....
processes from pre-existing formations that were originally either igneous

Igneous rock

Igneous rock is one of the three main Rock types . Igneous rock is formed by magma being cooled and becoming solid . They may form with or without crystallization, either below the surface as Intrusion rocks or on the surface as extrusive rocks....
or sedimentary

Sedimentary rock

Sedimentary rock is one of the three main Rock types . Sedimentary rock is formed by deposition and consolidation of mineral and organic material and from precipitation of minerals from solution....
rocks. Gneissic rocks are usually medium to coarse foliated

Foliation (geology)

Foliation is any penetrative planar Fabric present in Rock . Foliation is common to rocks affected by regional metamorphism compression typical of orogeny....
and largely recrystallized

Recrystallization

Recrystallization is a physical process that has meanings in chemistry, metallurgy and geology....
but do not carry large quantities of mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic with a tendency towards pseudo-hexagonal crystals and are similar in chemical composition....
s, chlorite

Chlorite group

The chlorites are a group of Silicate minerals minerals. Chlorites can be described by the following four Solid solution based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
or other platy mineral

Mineral

A mineral is a naturally occurring solid formed through Geology processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties....
s.

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Encyclopedia

Gneiss is a common and widely distributed type of rock

Rock (geology)

Metamorphic rock

Igneous rock

Sedimentary rock

Foliation (geology)

Foliation is any penetrative planar Fabric present in Rock . Foliation is common to rocks affected by regional metamorphism compression typical of orogeny....
and largely recrystallized

Recrystallization

Recrystallization is a physical process that has meanings in chemistry, metallurgy and geology....
but do not carry large quantities of mica

Mica

Chlorite group

Mineral

Granite

Granite is a common and widely occurring type of Intrusion , felsic, igneous rock rock . Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as Porphyry ....
gneisses, diorite

Diorite

Diorite is a grey to dark grey intermediate Intrusion igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene....
gneisses, etc. However, depending on their composition, they may also be called garnet

Garnet

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" comes from the Latin language granatus , possibly a reference to the Punica granatum , a plant with red seeds similar in shape, size, and color to some garnet crystals....
gneiss, biotite

Biotite

Biotite is a common Silicate minerals#Phyllosilicates mineral within the mica group, with the approximate chemical formula K3AlSi3O102....
gneiss, albite

Albite

Albite is a plagioclase feldspar mineral. It is the sodium Endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content....
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

Schist

The schists form a group of Erins metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, Chlorite group, talc, hornblende, graphite, and others....
, 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

Middle High German

Middle High German , abbreviated MHG , is the term used for the period in the history of the German language between 1050 and 1350. It is preceded by Old High German and followed by Early New High German....
verb

Verb

In syntax, a verb is a word that usually denotes an action , an occurrence , or a state of being . Depending on the language, a verb may vary in form according to many factors, possibly including its grammatical tense, grammatical aspect, grammatical mood and grammatical voice....
gneist (to spark; so called because the rock glitters) and has occurred in English

English language

English is a West Germanic language that originated in Anglo-Saxon England and has lingua franca status in many parts of the world as a result of the military, economic, scientific, political and cultural influence of the British Empire in the 18th, 19th and early 20th centuries and that of the United States from the mid 20th century onwa...
at least since 1757. Other sources claim the root to be an old Saxon

Saxony

The Free State of Saxony is a States of Germany of Germany. Located in the southeastern part of present-day Germany. It is the tenth-largest German state in area and the sixth largest in population , of Germany's sixteen states....
mining term that seems to have meant decayed, rotten, or possibly worthless material.

Augen gneiss

Augen gneiss is a coarse-grained gneiss, interpreted as resulting from metamorphism of granite, which contains characteristic elliptic or lenticular

Lens (geology)

In geology a lens is a body of ore or Rock or a deposit that is thick in the middle and thin at the edges, resembling a convex lens in cross-section....
shear bound feldspar porphyroclast

Porphyroclast

A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a groundmass of finer grained crystals. Porphyroclasts are fragments of the original rock before dynamic recrystallisation produced the groundmass....
s, normally microcline

Microcline

Microcline is an important igneous rocks-forming Silicate minerals mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium....
, within the layering of the quartz, biotite and magnetite bands.

Etymology: from the German Augen , meaning "eyes".

Gneiss

Augen gneiss

See also