CI chondrite

CI chondrites, and C1 - chondrites, are stony meteorites, which are among the carbonaceous chondrites. Their importance lies in their composition, which comes in closest to the sun among all meteorites found so far the element frequency distribution.

Designation

The term CI goes back to C for English Carbonaceous chondrites ( carbonaceous chondrites ) and I for the type locality Ivuna in Tanzania. The 1 in the C1 designation refers to the type 1 Van Schmus -Wood classification. The type 1 does not contain chondrules in the normal case.

Fund history

The previous findings of CI chondrites can be counted on one hand. The oldest find dates from 1806, two meteorites were then at Alais in France come down ( finds at Saint -Etienne and Valence ). They weighed a total of 6 kilograms. In 1864 a renewed decline in France near Orgueil at Montauban. The meteorite was divided into 20 fragments weighing a total of 10 kilograms. 1911, a meteorite was spotted in Tonk in India. There were several small fragments, but which weighed a total of only 7.7 grams. At the type locality in Tanzania Ivuna meteorite crashed 1938 to the ground, shattered into three 705 -gram fragments. 1965 The last incident occurred in Revelstoke, British Columbia ( Canada ) with two fragments that were only 1 grams but. Overall, therefore, there are only about 17 kilograms of CI chondrites.

Worth mentioning is the discovery of a meteorite in the course of the Apollo 12 mission (1969 ) on the moon. The Fund was initially thought to be a CI chondrite, but gave himself then as a closely related CM chondrite out. On Tagish Lake in the Yukon Territory in 2000 went down a meteorite, which is now counted among the CI chondrites. He surprisingly contains chondrules and is therefore performed as Cl 2.

Description

CI chondrites are quite fragile, porous rocks which break very easily when passing through the atmosphere. Why are only relatively small fragments. When Revelstoke case, although a huge ball of fire was seen, were found but as mentioned only two fragments of less than one gram! CI chondrites have a matte black fusion crust, which can often be very difficult to distinguish from their uniforms matrix inside. The opaque matrix is black in color due to the high content of carbon-rich material, fine-grained magnetite and ( minor ) pyrrhotite. You may also white-colored, lead hydrous carbonates and sulfates.

Main feature of the CI chondrites is the lack of clear chondrules (except Tagish Lake ). Tiny Chondrenfragmente and calcium - aluminum - rich inclusions ( CAI engl. ) but come before, albeit very rarely.

Mineralogical composition

CI chondrites contain in their matrix following minerals:

  • Olivine. Tiny, equal -sized, euhedral grains of forsterite ( with fayalite FA10 -20). Originated in the high temperature range.
  • Clinopyroxene. Also tiny equal -sized, euhedral grains. Originated in the high temperature range.
  • Orthopyroxene. Tiny, equal -sized, euhedral grains. Originated in the high temperature range.
  • Magnetite. Trained as framboids, spherulites and platelets.
  • Pyrrhotite.
  • Hydrous, clay-rich silicates such as montmorillonite and serpentine -like minerals.
  • Epsomite. As microscopic veins.

The ferro- magnesium minerals occur only in isolation and show, surprisingly, no detectable conversion phenomena Bem montmorillonite and the Serpentinähnlichen is, however, assumed that they are formed under water intake from magnesium-rich olivines and pyroxenes.

Chemical composition

CI chondrites contain between 17% and 22% water. The high porosity (up to 30 % ) is likely to be associated with this issue. But only in a bound form water occurs in hydrous silicates. The occurrence of epsomite indicates the presence of liquid water in the bedrock of the meteorite, which had penetrated the basis of cracks. At the cracks then the sulfates and carbonates were deposited.

The iron ( 25 weight percent ) is in CI chondrites practically only in the oxidized form, iron sulfides and elemental iron are as good as non-existent. The Mg / Si ratio is quite high at 1.07. Only CV chondrites are even more enriched in Mg. The Ca / Si ratio is lowest with 0.057 within the carbonaceous chondrites. However, the δ17O and δ18O values ​​are the highest among the carbonaceous chondrites. The ratio of two oxygen isotopes 17/18 corresponds to the values ​​found in terrestrial samples.

Physical parameters

CI chondrites have only a very low density of 2.2 g/cm3 for the high Prosität.

Importance

Of all meteorites found so far CI chondrites are similar to most of the element frequency distribution in the original solar nebula. They are therefore referred to as primitive meteorites. Except for the volatile elements carbon, hydrogen, oxygen and nitrogen, which occur depleted in CI chondrites, the frequencies are virtually identical. Lithium represents another exception, it is in the meteorites to the sun enriched (Lithium is used in the nucleosynthesis inside the sun consumed).

Of great importance is also the fact that in CI chondrites, with its very high carbon content in addition to the inorganic carbonates and graphite and organic carbon compounds are included - this is especially with regard to the discussion about the origin of life ( as there are for example, amino acids).

Normalization

Because of the similarities of the CI chondrites with the matter of the solar nebula rock samples are normalized in the petrology towards CI chondrites, ie, the ratio of sample / chondrite for the item in question formed. Ratios > 1 indicate an enrichment of the sample with respect to the solar matter to recognize ratios <1 depletion. This method is used mainly in the so-called Spiderdiagrams and primarily affects the lanthanides ( rare earths).

The element concentrations used in the normalization is as follows:

Terrestrial mantle and lower crustal rocks accumulate compared to CI chondrites with the exception of lanthanum to the rare earths. In return, CI chondrites can turn quite have higher values ​​of trace elements cerium, chromium, neodymium, strontium, vanadium and zirconium.

Origin

CI chondrites and the closely related CM chondrites are very rich in volatile substances, in particular water. It is therefore believed that they originated in the outer asteroid belt > 4 AE. For the former solar nebula made ​​this a critical distance is, the so-called snow line (English snowline ), beyond which water at 160 ° K to ice remained condensed and thus obtained. CI chondrites show in its composition actually a resemblance to the icy moons of the outer solar system. Furthermore, a direct relationship exists to comets: as well as they accreted the CI asteroids silicate minerals, ice and other volatiles, and organic compounds (see Halley's Comet ).

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