Peridotite

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

Composition

The chemical composition of peridotite is different depending on geotektonischem environment caused but in any case an ultramafic, so dominated by magnesium and iron silicates with olivine mineralogy as the dominant mineral. From kosmochemischen models and studies of ophiolites and xenoliths are representative chemical compositions of peridotite in different environments, in particular for "primitive", ie. original, non-depleted by melt processes of certain elements or minerals that have been constructed coat. Of particular importance here is Pyrolit, developed by Alfred Edward Ringwood theoretical composition of the primitive mantle.

The composition of peridotite in the uppermost 200-300 km of the bowl is changed especially by melt processes, primarily at mid-ocean ridges and in subduction zones. By melting of peridotite at mid-ocean ridges formed basaltic melt, which forms the oceanic crust. In these processes, the melt peridotite depleted of specific elements, in particular of iron, aluminum, calcium and sodium; the change of the molar ratio Mg / (Mg Fe ) is often indicated by the magnesium number Mg #. Its mineralogical composition is shifting to higher olivine and lower Pyroxengehalten out as shown in the following tables. Peridotite with more than 10 percent each orthopyroxene and clinopyroxene and the respective fourth main mineral is called lherzolite. Decreases by melting of the Klinopyroxengehalt below 10 percent, defined as the impoverished rock as harzburgite, also the Orthopyroxenanteil falls below 10 percent, as it is called now, more than 90 percent of olivine existing rock dunite. A peridotite with less than 10 percent orthopyroxene, but a higher proportion of clinopyroxene is, wehrlite.

The fourth main mineral in peridotite upper mantle is the main storage for aluminum. At pressures of less than about 0.9 GPa ( about 30 km depth ) is the plagioclase, between 0.9 and approximately 2.1 to 2.7 GPa ( about 60 to 85 km deep ) spinel, and at even higher pressures garnet. Due to the different chemical compositions of these minerals occur at the boundaries between the stability fields also to shifts in the proportions of other minerals. From about 350 km depth, and especially in the transition zone of the mantle 410-660 km there pyroxene and garnet combine to form aluminum- poor garnet and then while olivine converts to garnet - majorite to his high-pressure wadsleyite and ringwoodite forms. In the lower mantle, the mineralogy changes completely and includes perovskites and ferropericlase.

In addition to the above main minerals containing peridotite depending on the local chemical conditions even small amounts of other minerals whose existence depends in part on the content of volatiles. Especially in subduction zones, where the proportion of water in the mantle in parts per thousand or even percentage may be, minerals occur in the sum formula of water or hydroxyl ( OH) occurs, for example, amphibole or phlogopite. To graphite or diamond can form in carbon dioxide-rich environments. Volatile also affect the location of the melting point; as the water-saturated solidus of peridotite is several hundred degrees below the anhydrous.

Occurrence and use

Larger presence of unconverted peridotite are rare in Central Europe (mainly at subduction zones, eg in the Alps in the Val Malenco or Kraubath ) distributed, small xenoliths are as so-called Olivinbomben in basaltic rocks. Converted peridotites form part of the serpentinite and are much more common.

The few mineable deposits of Central Europe have no greater significance in terms of industrial use. In the time of the GDR was a Pikrit ( from the Peridotitfamilie with over 50 % share of olivine ) mined and used for various construction projects, such as the Dresden Palace of Culture in Thuringia. The steps in its main foyer and the side decks were made ​​of the Pikrit of Seibis at Lobenstein. In Russia there are several Pikrit deposits.

The rock is suitable for flooring and stairs ( Pikrit ). Due to its high density is expected from a good durability.

Dark peridotites (eg from South Africa) have been widely used for grave stones.

Peridotites and their weathering products ( serpentinites ) lead accumulations of chromium spinel ( chromite ), especially at the transition from dunitischen to resin- burg matic blocks. These enrichments were and are used as chrome ore (including Guleman in Turkey, Kokkinorotsos in Cyprus Troodos Mountains, in Sepentiniten of the Balkans in the 19th century even in Kraubath in Styria as Farberz ).

Under tropical conditions, weather lateritisch peridotites, and in the laterites are the low nickel contents (by 0.2-0.5 %) of the olivine then enriched (a few percent, in Noumeauit / garnierite much more). These then serve as a source of nickel laterites specifically for the production of ferronickel. At the tertiary educated Laterite nickel ore were mined as in the Saxon Granulitgebirge (Upper Call Berg) and the previous Jordansmühl in the Sudetes.

Carbon dioxide storage

Researchers at New York's Columbia University have demonstrated that in certain peridotite rocks can - chemical reactions take place, absorb carbon in the rock in the form of carbonates. The researchers believe it is possible to bind in peridotite - rocks overlying yet to be developed technical methods billion tons of carbon dioxide and thus remove them from the atmosphere. Carbon dioxide is one of the main cause of global warming.