Peridotite

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Peridotite xenolite from San Carlos (southwest USA). The rock is typically rich in olivine, crossed by a centimeter-thick layer of green-black pyroxenite.

Peridotite is a coarsely crystalline ultramafic rock that makes up most of the earth's mantle . Peridotite contains at least 40 percent olivine ; the rest is essentially composed of orthopyroxene , clinopyroxene and an aluminum-containing mineral - depending on the pressure and temperature, garnet , spinel or (rarely) plagioclase . The name comes from the French mineral name peridot .

composition

Chemical composition of peridotite in% by weight
material MORB pyrolite Lherzolite Harzburgite Dunite
SiO 2 44.74 44.16 43.8 41.3
MgO 39.57 41.05 46.3 51.88
FeO 7.55 8.14 7.71 6.26
Al 2 O 3 4.37 2.25 0.56 0.1
CaO 3.38 2.27 0.75 0.16
Cr 2 O 3 0.45 0.39 0.38 0.28
Na 2 O 0.4 0.21 0.07 0.032
NOK 0.26 0.27 0.35 0.36
TiO 2 0.17 0.09 0.08 0
MnO 0.11 0.14 0.12 0.08
K 2 O 0 0.02 0.03 0.02
P 2 O 5 0 0.03 0.01 0.05
Mg # 90.0 90.0 91.4 93.7
Mineralogical composition (in% by weight) of peridotite
material primitive Lherzolite Harzburgite Dunite
Spinel peridotite
Olivine 56 62 81 98
Orthopyroxene 22nd 22nd 14th -
Clinopyroxene 19th 11 2 1
Spinel 3 2 3 1
Garnet peridotite
Olivine 57 68 83 98
Orthopyroxene 16 18th 15th -
Clinopyroxene 14th 11 - 1
garnet 13 3 2 1

The chemical composition of peridotite differs depending on the geotectonic environment, but in any case it causes an ultramafic mineralogy characterized by magnesium and iron silicates with olivine as the dominant mineral. From cosmochemical models as well as studies on ophiolites and xenolites , representative chemical compositions for peridotite in various environments, especially also for "primitive", i.e. H. original jacket, not depleted of certain elements or minerals by melting processes. Of particular importance here is pyrolite , a theoretical composition of the primitive mantle developed by Alfred Edward Ringwood .

The composition of peridotite in the uppermost 200 to 300 km of the mantle is changed in particular by melting processes, primarily under mid-ocean ridges and in subduction zones . The melting of peridotite under mid-ocean ridges creates basaltic melt, which forms the oceanic crust. During these melting processes, the peridotite is depleted of certain elements, especially iron , aluminum , calcium and sodium ; the change in the molar ratio Mg / (Mg + Fe) is often indicated by the magnesium number Mg #. Its mineralogical composition shifts towards higher olivine and lower pyroxene contents, as the following tables show. Peridotite with more than 10 percent orthopyroxene and clinopyroxene each and the respective fourth main mineral is called lherzolite . If the clinopyroxene content falls below 10 percent due to melting, the impoverished rock is referred to as Harzburgite , and if the orthopyroxene content falls below 10 percent, the rock , which now consists of more than 90 percent olivine, is called dunite . A peridotite with less than 10 percent orthopyroxene but a higher proportion of clinopyroxene is called wehrlite .

The fourth main mineral in the peridotite of the upper mantle is the most important reservoir for aluminum. At pressures of less than approx. 0.9 GPa (approx. 30 km depth) this is plagioclase , between 0.9 and approx. 2.1 to 2.7 GPa (approx. 60 to 85 km depth) spinel and at even higher pressures Garnet . Due to the different chemical compositions of these minerals, there are also shifts in the proportions of the other minerals at the boundaries between the stability fields. From a depth of around 350 km and especially in the transition zone of the earth's mantle between 410 and 660 km, pyroxene and garnet combine to form low-aluminum garnet and then to garnet majorite , while olivine transforms into its high-pressure forms wadsleyite and ringwoodite . In the lower mantle, mineralogy changes completely and includes perovskites and ferropericlase .

In addition to the main minerals mentioned above, peridotite also contains small amounts of other minerals, depending on the local chemical conditions, the existence of which depends in part on the volatile content. Particularly in subduction zones, where the proportion of water in the mantle can be in the per mille or even percentage range, minerals occur in the sum formula of which water or hydroxyl (OH) occurs, e.g. B. amphibole or phlogopite . Graphite or diamond can form in environments rich in carbon dioxide . Volatiles also influence the position of the melting point; the solidus temperature of water-saturated peridotite is several hundred degrees below that of anhydrous.

Occurrence and use

Larger occurrences of unconverted peridotite are rare in Central Europe (mainly in subduction zones, in the Alps e.g. in Val Malenco or near Kraubath an der Mur ), small xenolites are common as so-called olivine bombs in basaltic rocks. Converted peridotites form part of the serpentinites and are much more common.

The few Central European deposits that can be mined are of no great importance in terms of industrial use. In the time of the GDR, a picrite (from the peridotite family with over 50% olivine) was mined in Thuringia and used for various building projects, such as the Dresden Palace of Culture . The stairs in his main foyer and in the side staircases were made from the Pikrit from Seibis near Lobenstein. There are several picrite deposits in Russia.

The stone is suitable for floor coverings and stairs (picrit). Due to its high density, a good shelf life can be assumed.

Dark peridotites (e.g. from South Africa) were often used for tombstones.

Peridotites or their weathering products (serpentinites) also lead to accumulations of chromium spinel ( chromite ), especially at the transition from dunite to resin burgite clods. These enrichments were and are used as chrome ore (including Guleman in Turkey, Kokkinorotsos in the Troodos Mountains in Cyprus, in Sepentinites of the Balkans, in the 19th century even as color ore near Kraubath in Styria).

Under tropical conditions, peridotites weather lateritically, and the low nickel contents (around 0.2–0.5%) of olivine are then enriched in the laterites (a few percent, in Noumeauit / Garnierite much more). These laterites then serve as a source of nickel specifically for the production of ferronickel. Laterites formed in the tertiary were mined as nickel ore in the Saxon granulite mountains (Obercallenberg) and at the former Jordansmühl in the Sudetes.

Carbon dioxide storage

Researchers at New York's Columbia University were able to show that chemical reactions can take place in certain peridotite rocks, which bind carbon dioxide in the rock in the form of carbonates. The researchers believe it is possible to use technical methods that have yet to be developed to bind billions of tons of carbon dioxide in peridotite rocks and thus remove them from the earth's atmosphere. Carbon dioxide is considered to be a major contributor to global warming .

literature

  • DH Green, TJ Falloon: Pyrolite: A Ringwood concept and its current expression. In: I. Jackson (Ed.): The Earth's Mantle - Composition, Structure, and Evolution. Cambridge University Press, 1998, ISBN 0-521-78566-9 , pp. 311-378 (chemical composition MORB pyrolite).
  • WF McDonough, RL Rudnick: Mineralogy and composition of the upper mantle . In: Reviews in Mineralogy and Geochemistry . tape 37 , no. 1 , 1998, p. 139–164 (chemical and mineralogical composition).
  • Arndt Peschel: natural stones. 2nd edition, Leipzig 1977.
  • Wolfhard Wimmenauer : Petrography of igneous and metamorphic rocks , Stuttgart 1985.
  • Walther E. Petrascheck , Walter Pohl: deposit theory. 3rd edition, E. Schwizerbarth'sche Verlagsbuchhandlung, Stuttgart 1982, ISBN 3-510-65105-7 .

Web links

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

swell

  1. ^ KA Redlich, K. v. Terzaghi, R. Kampe: Engineering Geology . Springer-Verlag, Vienna 1929, ISBN 978-3-7091-5996-5 , p. 44 . ( limited preview in Google Book search)
  2. Hans Leitmeier: Introduction to rock science . Springer-Verlag, Vienna 1950, ISBN 978-3-7091-3606-5 , p. 74 . ( limited preview in Google Book search)
  3. The Gulsen - Mineralogy. Market town of Kraubath an der Mur, accessed on October 10, 2015 .
  4. a b Peter B. Kelemen and Jürg Matter: In situ carbonation of peridotite for CO2 storage . In: Proceedings of the National Academy of Sciences . 2008 [1]