Granulite

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Granulite (from Latin granum = "grain") are highly metamorphic, crystalline slates with little water of crystallization. A distinction is made between the following types of granulite: light granulite (white stone granulite), granular granulite, eye granulite and pyroxene and corundum granulite.

Origin and chemical composition

Pressure-temperature-depth diagram of metamorphic facies areas, 6 = granulite facies

→ see also: Granulite facies
Granulites are highly metamorphic rocks whose light-colored minerals have roughly the composition of granites, but whose dark silicates are largely free of hydroxyl groups . Educts are, on the one hand, acidic magmatites (rhyolithoids) and their tuffs. On the other hand, sandy slate, greywacke and arkose can also be considered, which are exposed to very high pressures and the absence of volatile components (water) in the deepest tectonic mountain layers. Due to the strong (often multi-phase) metamorphosis and the associated homogenization, the original rocks are usually difficult to reconstruct. For the granulites of the Saxon granulite mountains , due to quartzitic, Al 2 O 3 -rich as well as calcium silicate and metabasite deposits, at least a strong participation of sedimentary material in the educt must be assumed. As the last (pre-granulitic) metamorphic stage, a migmatitic gneiss complex can be reconstructed.

The following table shows the chemical composition of granulites (in% by mass)

chem. connection 1 2 3 4th
SiO 2 73.9 66.7 70.4 68.1
Ti 2 O 2 0.2 0.6 0.3 0.7
Al 2 O 3 14.2 15.7 14.5 15.2
Fe 2 O 3 <0.1 0.1 0.4 0.2
FeO 1.8 4.3 3.1 3.8
MnO 0.02 0.02 0.16 0.07
MgO 0.4 1.4 1.2 1.6
CaO 1.3 3.4 1.3 1.7
Na 2 O 2.7 3.5 4.0 2.9
K 2 O 4.4 2.7 4.2 3.9
H 2 O + <0.1 0.6 0.4 0.7
CO 2 - - - -
swell

1 = "White stone" granulite, Tirschheim borehole (Saxon Granulite Mountains)
2 = Granulite containing pyroxes , large cities, (Saxon Granulite Mountains)
3 = Granulite, Dunkelsteiner Forest (Lower Austria)
4 = Granulite, Olši ( Moravia , Czech Republic )

Mineral inventory

The main components of granulite are feldspar and quartz in varying proportions, which are present in granoblastic grain association. There are hardly any hydroxyl-containing iron-manganese silicates. Typical accompanying minerals are ferrosilite- rich orthopyroxene , almandine-rich garnet , thistle and sillimanite . Spinel , rutile , ilmenite and graphite often appear as excess particles . The feldspar of the granulite is mostly perthitic or antipertitic. The segregation bodies in perthite are often in the form of hair- thin lamellae (hair perthite) or in such large quantities that it is difficult to assign the primary feldspar mixed crystal to potassium feldspar or plagioclase (mesoperthite). Due to the low proportion of mafic minerals, granulites are usually light in color. Quartz-potassium feldspar-rich types can appear almost white ("white stone"). Even small amounts of finely divided (secondary) biotites , orthopyroxenes or spinels tend to give the granulites a darker color.

The following table summarizes the mineralogical composition of some granulite representatives (in% by mass).

mineral 1 2 3 4th
quartz 40 38 60 48
Potassium feldspar 55 57 18th 33
Plagioclase - - 12 11
garnet 3 3 5 2
Disthene / sillimanite 2 0.5 0.2 3
Acczesories - 0.5 1 -
Diaphthoresis products (biotite, muscovite) - 1 3.8 3
swell

1 = "White stone" granulite, Saxon granulite mountains (mean)
2 = light, thick schistiger granulite from the Saxon granulite mountains (normal formation)
3 = granulite (mean), Meidling , Lower Austria
4 = granulite, Olši (Moravia), Czech Republic

structure

The mesoscopic appearance of the granulite is roughly comparable to that of gneiss: a layered to slate parallel texture is common. Many granulites are small to medium-grained, there are also thick schisty to massive varieties ("granofels"). The s-faces are marked by lenticular to disk-shaped quartz grains. Thin-slate rock types that occur within or on the edge of granulite complexes are mostly retrograde descendants of granulites. The s-surfaces are emphasized here by wallpaper-like or line-like recrystallized quartz and / or secondary mica. Granulites converted in this way are then also referred to as metagranulitic biotite gneisses, granulite gneisses or biotite granulites .

Occurrence

In Central Europe, granulites occur in various areas of the Bohemian Massif . The best-known occurrences are those of the Saxon Granulite Mountains, the Czech and Lower Austrian Moldanubic and the Owl Mountains in southern Poland.

While these occurrences and most of the other European and non-European occurrences are attributed to Precambrian metamorphic epochs, a Variscan metamorphic age can be assumed for the small granulite occurrence of Zöblitz ( Ore Mountains ) . A bond to tectonic shear zones is characteristic of all granulite deposits in Central Europe.

Physico-technical properties and use

Due to their high compressive strength of up to 300 N / mm² and their high abrasion resistance, granulites are suitable as road building materials for packing stores, small pavement and gravel as well as railway gravel. Granulite is easy to saw, and together with its frost resistance, it is therefore used as paving slabs, facing and decorative stones in outdoor areas.

The following table provides an overview of some technical and physical properties.

Bulk density 2.60 - 2.75 kg / dm³
Compressive strength 130-300 N / mm²
Flexural strength 12-25 N / mm²
Water absorption 0.2-0.5% by weight
Abrasion resistance 4 - 10 cm³ / 50 cm²

literature

  • R. Weber, D. Hill: Natural stone for users. Ebner Verlag, Ulm 2008, ISBN 978-3-87188-108-4 .
  • W. Wimmenauer: Petrography of igneous and metamorphic rocks. Enke-Verlag, Stuttgart 1985, ISBN 3-432-94671-6 .
  • L. Pfeiffer, M. Kurz, G. Mathé: Introduction to Petrology. 2., arr. Edition. Akademie-Verlag, Berlin 1985, DNB 860602648 .
  • R. Jubelt, P. Schreiter: Rock determination book. 4th, through Edition. VEB German publishing house for basic industry, Leipzig 1977, DNB 770228585 .
  • A. Watznauer, H.-J. Behr, G. Mathé: The granulites of Saxony. In: Freiberg research books. C. 268, Leipzig 1971, pp. 91-111.
  • HG Scharbert: Cyanite and sillimanite in Moldanubian granulites. In: Tschermaks min. petrogr. Mitt. 16, 4, Vienna 1971, pp. 252-267.
  • O. Matějovska: Composition of coexisting garnet and biotite from some granulites of Moldanubicum Czechoslovakia. In: New yearbook for mineralogy monthly books. Stuttgart 1970.
  • HG Scharbert: On the nomenclature of the rocks of the granulite facies. In: Tschermaks min. petrogr. Mitt. 8, 4, Vienna 1963.

Individual evidence

  1. ^ R. Jubelt, P. Schreiter: Rock determination book. VEB Deutscher Verlag for basic industry, Leipzig 1977, pp. 90, 91.
  2. ↑ a b c d e f L. Pfeiffer, M. Kurz, G. Mathé: Introduction to Petrology. Akademie-Verlag, Berlin 1985, pp. 501–503.
  3. ↑ a b A. Watznauer, H.-J. Behr, G. Mathé: The granulites of Saxony. In: Freiberg research books. C. 268, Leipzig 1971, pp. 91-111.
  4. ↑ HG Scharbert: kyanite and sillimanite in Moldanubian granulites. In: Tschermaks min. petrogr. Mitt. 16, 4, Vienna 1971, pp. 252-267.
  5. ↑ a b O. Matějovska: Composition of coexisting garnet and biotite from some granulites of Moldanubicum. Czechoslovakia. In: New yearbook for mineralogy monthly books. Stuttgart 1970, pp. 249-263.
  6. ↑ HG Scharbert: On the nomenclature of the rocks of the granulite facies. In: Tschermaks min. petrogr. Mitt. 8, 4, Vienna 1963, pp. 591-598.
  7. ^ W. Wimmenauer: Petrography of igneous and metamorphic rocks. Enke-Verlag, Stuttgart 1985, p. 234.