Gneiss

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Banded gneiss
Gneiss quarry on the edge of the Palatinate Forest
The Hexenstein is a block of gneiss in Lake Constance near Lindau

The gneisses (old Saxon miner's name from the 16th century, perhaps in Old High German gneisto , Middle High German ganeist (e) , g (e) neiste "spark") are metamorphic rocks with a parallel texture that contain more than 20% feldspar . In the past (sometimes until the early 19th century), common names were: Gneus or Geneus .

Origin, occurrence and composition

Gneisses arise through metamorphosis , i. H. Transformation of rock under high pressure and at high temperatures. The starting material is covered by many layers of rock. As a result, gneiss only comes to the surface when either the overlying material has eroded or formerly deep layers have been lifted to the surface by tectonics .

Gneisses are widespread worldwide and are often found in the old cores ( cratons ) of the continents, where they were exposed by deep erosion . As a rule, these rocks have undergone several phases of transformation ( regional metamorphoses) since their formation . They form the oldest rock formations on earth, for example the Acasta gneiss from the Hadaikum of (4,030 mya) in the western Canadian shield and the gneiss-containing Nuvvuagittuq greenstone belt in the eastern Canadian shield. Gneisses also occur in the crystalline zones of younger fold mountains , e.g. B. in the main Alpine ridge of the Central Alps .

The composition of the gneiss depends on the source rock : the most important minerals in the gneiss do not form during metamorphosis, but are already present in the source material. Gneisses mainly consist of the mineral components listed in the following table (in%).

Gneiss variety quartz Potassium feldspar Plagioclase Biotite Muscovite Cordierite garnet Sillimanite other Density
(g / cm³)
Granodiorite gneiss 30th 8th 38 9 1 2.73
Gray gneiss 33 3 37 17th 9 1 2.71
Biotite plagioclase gneiss 31 4th 38 14th 12 1 2.70
Long-fiber red gneiss 39 27 25th 2 6th 1 2.67
Thin-plate

Red gneiss

43 29 13 1 12 2 2.63
Quartz eye gneiss 41 13 28 5 12 1 2.65
Aplit gneiss 44 11 29 15th 1 2.64
Flat, fine-grained paragneiss 14th 22nd 18th 40 6th 2.70
Layered two-mica gneiss 16 31 31 20th 2 2.68
Metatectical

Gray gneiss

29 1 66 1 2 1 2.72
Cordierite gneiss 20th 25th 40 13 2 2.74
Sillimanite gneiss 14th 18th 1 26th 14th 14th 4th 20th 6th
Garnet gneiss 13 7th 3 6th 27 35 6th

The structure fluctuates between fine and coarse-grained with a sometimes clearly visible parallel texture. In the latter case, we speak of regulated crystals . The rock can then appear layered, fibrous to coarse shale and often noticeably banded. In contrast to this, there are orthogneiss with poorly developed texture features and relatively uniform crystal sizes or only slightly crystallinity in the matrix .

Classification

Eye gneiss. Rio de Janeiro , Brazil

Gneisses differ in their mineral composition, in their structure and in their genesis (history of origin).

Mineralogically , the gneisses differ according to the type of minerals present. Differentiated according to different mica we know, for example, the biotite gneiss , the muscovite gneiss or the two-mica gneiss consisting of both mica mentioned . If there is a high proportion of cordierite or hornblende , one speaks of cordierite gneiss or hornblende gneiss .

Regardless of this, gneisses can also be distinguished on the basis of their structure : for example, eye gneisses are known when a fine-grained rock matrix "flows around" larger mineral inserts , or flaser gneisses when the structure is linear and fibrous and less planar and slate. Gneisses are often clearly banded.

Slate gneisses ( mica slate ) with strong movements are grouped under the collective name gneiss phyllite .

It is also common to differentiate between the gneisses and their parent rock (educts).

  • Orthogneiss is the metamorphic conversion product of feldspar and quartz-rich igneous rocks such as B. granite or granodiorite . Often, however, they have already undergone several rock transformations (polymetamorphosis) and emerged from existing gneisses.
Paragneiss, Serizzo

Granite gneiss , syenite - or debris gneiss they differ from chemical and mineralogical identical metamorphic rocks (such Metagranit etc.) only by its typical slate-like structure. Gneiss are also differentiated according to the degree of their metamorphosis as epi , meso or kata gneiss . The higher the pressure and the higher temperatures prevailed when they were created.

The layer structure ( foliation ) of the gneisses is created by segregation (segregation) of layers of light feldspars, quartz and dark minerals. This happens because of the different plasticity at different temperatures. Layered silicates (especially biotite ) tend to regulate themselves over a large area under tectonic stress through lateral recrystallization. Quartz and feldspars tend to remain grainy. The spatial position of the cleavage areas corresponds to the direction of the maximum tectonic shear forces .

Gneiss as a natural stone

properties

Examples of structural images of gneiss (polished) from the Pennine nappes between Locarno and Domodossola

Often, gneisses are also traded as "granite" because they have very similar technical properties to this rock group. However, they have a consistently higher water absorption and better flexural tensile values . As natural stones, they can be polished and are largely frost-resistant. The feldspars and mica determine the color, and quartz determines the abrasion resistance of the gneisses. Layered silicates give the gneisses their ability to cleave when they occur in layers in the rock.
Essential technical-physical parameters are summarized in the following table using the example of Erzgebirge gneiss.

True density (g / cm³) Bulk density (g / cm³) Water absorption (M% after 72 h storage ) Pressure pot strength (M%) 5 mm sieve passage Abrasion resistance (M%) 5 mm sieve passage Frost resistance (M%) <2 mm after 25 freezing Cylinder compressive strength (g / cm³)
2.70-2.79 2.65-2.72 0.4-1.3 8-16 17-24 0.1-0.8 1,100

The technical differences between para and orthogneiss are minimal. As a rule, the colored gneisses are either migmatite or orthogneiss, and the gray gneisses are paragneiss. The light (almost white) "gneisses" can also be granulites .

Well-known types of natural stone

Hallandia (Migmatite from Sweden)

Others

The professional association of German geoscientists has named gneiss Rock of the Year 2015.

See also

literature

  • Siegfried Matthes, Martin Okrusch: An introduction to special mineralogy, petrology and deposit science . Springer, Berlin 2005, ISBN 3-540-23812-3 .
  • Roland Vinx: Rock determination in the field . Springer, Berlin, Heidelberg 2008, ISBN 978-3-8274-1925-5 .
  • Gregor Markl: Minerals and Rocks. Properties - Education - Investigation . Elsevier, Munich 2004, ISBN 3-8274-1495-4 .
  • Hans Murawski: Geological Dictionary . Elsevier, Munich 2004, ISBN 3-8274-1445-8 .

Web links

Commons : Gneiss  - collection of images, videos and audio files

Remarks

  1. ↑ Jubelt, R., Schreiter, P., Rock determination book, VEB Deutscher Verlag für Grundstoffindustrie Leipzig 1977, page 84
  2. ^ The Geological Structure of Austria, p. 336 right column
  3. ↑ O. Wagenbreth; Basic scientific knowledge for civil engineers, technical rock science, VEB Verlag für Bauwesen Berlin 1977.
  4. ↑ LBEG Lower Saxony: Rock of the Year 2015: Just in time for the honor, huge gneiss appears. Press release on January 13, 2015