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Gneiss /ˈnaɪs/ 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. It is often foliated(composed of layers of sheet-like planar structures). The foliations are characterized by alternating darker and lighter colored bands, called "gneissic banding".

Contents

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• 1 Etymology
• 2 Composition
  • 2.1 Gneissic banding
• 3 Types
  • 3.1 Lewisian
  • 3.2 Augen gneiss
  • 3.3 Archean and Proterozoic gneiss
  • 3.4 Henderson Gneiss
• 4 See also
• 5 Footnotes
• 6 References
• 7 External links

Etymology[edit]

The etymology of the word gneiss is disputed. Sources say it comes from the Middle High German verb gneist (to spark; so called because the rock glitters). It has occurred in English since at least 1757.^[1]

Composition[edit]

Gneissic rocks are usually medium- to coarse-foliated and are 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. Gneiss rocks may also be named after a characteristic component, such as garnet gneiss, biotite gneiss, albite gneiss, etc. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock.

Not all gneiss rocks have detectable banding however. Kyanite gneiss crystals look like random clumps of kyanite in what is mainly a plagioclase (albite) matrix.

Gneissic banding[edit]

Gneiss appears to be striped in bands, called gneissic banding.^[2] The banding is developed under high temperature and pressure conditions.

The minerals are arranged into layers which appear to be bands, when the rock is broken and viewed from the side (in cross section).^[2] This is because of a different composition of each layer, or band, called compositional banding. The darker bands have relatively more mafic minerals (those containing relatively more magnesium - Mg, and iron Fe). The lighter bands contain relatively more felsic minerals (silicates, containing relatively more of the lighter elements, such as silicon, oxygen, aluminium, sodium, and potassium).

A common cause of the banding is the subjection of the protolith (the original rock material that undergoes metamorphism) to extreme shearing force, a sliding force similar to the pushing of the top of a deck of cards in one direction, and the bottom of the deck in the other direction.^[2] These forces stretch out the rock like a plastic, and the original material is smeared out into sheets.

Some banding is formed from original rock material (protolith) that is subjected to extreme temperature and pressure and is composed of alternating layers of sandstone (lighter) and shale (darker), which is metamorphosed into bands of quartzite and mica.^[2]

Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood.^[2]