Gneiss

The gneisses ( old Saxon miner name from the 16th century, perhaps to Old High German: gneisto, Middle High German: ganeist ( e), g (s) neiste " spark" ) are metamorphic rocks with parallel texture, containing more than 20 % feldspar. Previously (partly still in Goethe ) was the common form of the name Gneus.

Formation, occurrence and composition

Gneisses occur through metamorphosis, ie at relatively high temperature-pressure conditions; while the raw material of many rock layers is superimposed. On the surface, gneiss is therefore only exists when either the opposing material is eroded or formerly low-lying layers were lifted by tectonics to the surface.

Gneisses are found worldwide and are often found in the old nuclei ( cratons ) of the continents, where they were exposed by deep erosion. In general, these rocks have participated since its inception several stages of rock alteration (Regional Metamorphoses ). They form the oldest rock formations on the planet, so the Acasta - gneiss from the Hadean of 4,030 mya containing from western Canadian Shield and the gneisses Nuvvuagittuq greenstone belt from the eastern Canadian Shield. Gneisses also occur in the crystalline zones younger fold mountains, such as the Alpine main ridge of the central Alps.

The composition of the gneisses depends on the parent rock from: The main minerals in the gneiss form not during metamorphosis, but are already in the starting material. Gneiss is mainly composed of the minerals feldspar (> 20% ), quartz and mica, these are in addition to the calcite, in the continental crust widespread minerals. The texture varies from fine - and coarse-grained, sometimes in a conspicuous parallel texture. In the latter case one speaks of the controlled crystals. The rock can then ply - flaserig to grobschieferig, and often strikingly banded appear. In contrast, there are orthogneiss with weakly developed texture features and relatively uniform crystal sizes or in the matrix only slightly more pronounced crystallinity.

Classification

Gneisses can be due to their mineral composition, divided according to their structure, or after their formation ( genesis ).

Mineralogical the gneisses differ by type of existing minerals. Distinguished according to mica is known as the Biotitgneis, the Muskovitgneis or consisting of two said micas Two mica gneiss. At high proportions of cordierite or hornblende one speaks accordingly of Cordieritgneis or hornblende gneiss.

Regardless, you can gneisses also divided on the basis of their structure: so one knows eyes gneisses, when a fine-grained rock matrix larger mineral Einsprenglige "flows " or Flasergneise when the structure is linear manner flaserig - developed and less areally - schist. Often gneisses are clearly banded.

Stark by moving slate gneiss to mica schist are grouped under the collective term Gneisphyllite.

Usual is also a distinction between the gneisses by their parent rocks ( reactants ):

• Orthogneisses represent the metamorphic alteration product of feldspar and quartz - rich igneous rocks, such as Granite or granodiorite dar. Often, but they have already gone through several transformations rock ( Polymetamorphose ) and originated from existing gneisses.

• Paragneiss caused by the conversion of sedimentary rocks ( sandstones, greywackes, arkoses and shales ), and therefore often have a greater variety of accessory minerals (low batch parts) as the gneiss.

• A transition from the metamorphic gneisses to the igneous plutonic rocks represents the migmatite or Anatexit

If the parent rock of gneiss is accurately known, is spoken by granite - gneiss, syenite gneiss or gneiss boulders. In this they differ from chemically and mineralogically identical metamorphic rocks, such as meta- granite, etc. only by their typical slate-like structure. In addition, gneisses can also be classified according to their degree of metamorphism, as epi-, meso -, or Kata - gneiss, with each higher pressure and higher temperatures in the formation.

The layer structure ( foliation ) of the gneisses is caused by the segregation ( segregation ) of layers of bright feldspars and quartz and dark minerals, due to their different plasticity at high temperatures. In particular, the layer silicates (especially biotite ) tend to regulate surface under tectonic stress by lateral Neukristallisation while quartz and feldspars remain rather grainy. The spatial position of the foliation surfaces corresponds to the direction of maximum tectonic shear.

Gneiss than natural stone

Often gneisses are traded as " granite ", since they have very similar technical characteristics as this rock group. However, they have a consistently higher water absorption and better Biegezugwerte. They can be polished as natural stones and largely resistant to frost. The feldspar and mica and quartz define the color determines the abrasion resistance of the gneisses. Phyllosilicates give the gneisses their good capability gap when they occur in the plant -shaped rock.

The technical differences between para-and ortho- gneisses are negligible. It can be said orienting that are usually colorful gneisses or orthogneisses and migmatites either the gray paragneiss. The bright (almost white ) " gneiss " can also be granulites.

Known types of natural stone

• Hallandia, migmatite (Sweden), Halmstad
• Juparana Classico, orthogneiss (Brazil, Rio de Janeiro)
• Midnight Blue, orthogneiss (India)
• Orissa, orthogneiss (India)
• Steinbacher augen gneiss, orthogneiss ( Germany, Steinbach )
• Serizzo Antigorio paragneiss ( Switzerland / Kt. Ticino, Valle Antigorio )
• Calanca, paragneiss ( Switzerland / Kt. Graubünden, Calanca Valley )
• Onsernone, paragneiss ( Switzerland / Canton of Ticino, Onsernone Valley )
• Maggia, paragneiss ( Switzerland / Canton of Ticino, the Maggia Valley )
• Soglio, paragneiss ( Switzerland, Soglio )
• Branco Ipanema, granulite (Brazil, Espirito Santo )
• Stainzer hard gneiss ( Austria, Vienna)
• Verde Andeer, ( Switzerland / Kt. Graubünden, Andeer )
• Please shear gneiss granite gneiss, Austria