Chondrite

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Allende (meteorite)
L6 chondrite Holbrook. Cut surface, light grains are metal. Height 5 cm.

With a share of around 86 percent, chondrites form the largest class of meteorites . Your name comes from trapped small silicate beads ago, called chondrules , which in a fine grained matrix are embedded. The mineralogical composition of the chondrites is dominated by the minerals olivine , pyroxene and plagioclase . But they also always contain metallic nickel iron (see picture Chondrite Holbrook) and iron sulfide ( Troilit ) (with a few exceptions in the case of the carbonaceous chondrites ).

Chondrites can be understood as cosmic sedimentary rocks . They are often also called undifferentiated meteorites, since their chemical composition, with the exception of gaseous and highly volatile elements such as sodium and potassium or the noble gases , corresponds to the composition of the photosphere of the sun , and thus that of the original solar nebula . Age determinations by measuring radioactive isotopes have shown that chondrites were formed in the early days of the solar system 4.5 billion years ago.

history

The fall of Lucé chondrites in France in 1768 was investigated by the French chemist Antoine Laurent de Lavoisier . At that time, however, the existence of meteorites was not universally recognized and Lavoisier declared the stone to be a type of iron gravel. However, the conspicuous chondrules that were observed in stones from the most varied of sites ultimately led to their extraterrestrial origin being recognized. In 1864, the famous French chemist Louis Pasteur also examined a meteorite that was assigned to the carbonaceous chondrites.

classification

NWA 3118, carbonaceous chondrite, CV3

Chondrites can be further divided into subclasses. The common chondrites come first in terms of frequency. The carbonaceous chondrites are so named because they contain larger amounts of carbon (also than organic compounds ). Their fracture surface therefore looks more or less coal black. In addition to chondrules, presolar minerals and calcium-aluminum-rich inclusions can also be found in their fine-grained matrix .

According to their chemical composition, the chondrites are divided into the following subclasses:

  • Ordinary chondrites . According to the total iron content and the content of nickel iron, they are divided into:
    • H -Chondrite (for H igh iron), with 22-30% total iron and 17-23% metal,
    • L -Chondrite (for L ow iron), with 20-24% total iron, and 4-9% metal,
    • L L -Chondrite (for L ow iron, L ow metal), with 19-22% total iron and 0.3-3% metal.

In the practical determination of the class, the iron oxide content in the olivine is used; it is inversely proportional to the total iron and amounts to: For H-chondrites 16–19% Fa (fayalite or mol% Fe / (Fe + Mg)), for L-chondrites 21-25% Fa, for LL-Chondrites 26-32% Fa. In the olivines of the unequilibrated type 3 chondrites (see below) the Fa content is variable between 0 and 50% Fa.

  • Enstatite chondrites . They do not contain olivine, and in pyroxene (here enstatite) the Fs content (ferrosilite or mol% Fe / (Fe + Mg)) is less than 1.
    • EH chondrites
    • EL chondrites
  • Coal chondrites (Primitive Chondrites)
    • CI (Ivuna Group)
    • CO (Ornans Group)
    • CV (Vigarano Group)
    • CM (Mighei Group)
    • CK (Karoonda Group)
    • CR (Renazzo Group)
    • CH (metal kingdom)
    • CB (Bencubbinite)
  • R-Chondrites ( Rumuruti-Chondrites )
  • K-Chondrites ( Kakangari-Chondrites )
  • F-chondrites ( forsterite chondrites)

An overview of the frequency of the elements in the various chondrite classes can be found in Kallemyn et al. ("Geochemistry of ordinary chondrites", Geochimica et Cosmochimica Acta, 1989, page 2747) can be found.

In addition to chemical classification, chondrites are divided into petrological types 1 to 6, sometimes also 1 to 7, according to a scheme proposed by the scientists Van Schmus and Wood in 1967 . Petrological type 3 chondrites are called "unequilibrated" because they represent material that remained almost unchanged after the formation of the mother body through accumulation from the solar mist. In contrast, type 4 to 6 chondrites were increasingly thermally metamorphosed . These chondrites are recrystallized. As a result, in type 6 chondrites, the boundaries between the chondrules and the matrix are blurred (see the picture of the L6 chondrite Holbrook). Type 1 and 2 meteorites have not been thermally changed, but have undergone an "aqueous" metamorphosis and contain silicates ( clay minerals ) containing water of crystallization . While type 3 to 6 chondrites contain a maximum of 3 percent by weight of water, type 2 chondrites can contain up to 18 percent by weight and type 1 chondrites more than 20 percent by weight of water. Type CI1 chondrites do not contain any chondrules, although it is not clear whether they ever had chondrules or whether they were destroyed by the aqueous alteration.

In common chondrites and entstatite chondrites, petrological types 3 to 7 occur, and in carbonaceous chondrites, petrological types 1 to 6 (up to CK5 / 6) occur.

Chondrites with different petrological types can certainly come from the same mother body. The "onion skin model" for the mother body was proposed for the H chondrites. According to this model, the mother body of the H-chondrite would be an undifferentiated asteroid , which after its formation 4.56 billion years ago was heated again by the decay of radioactive nuclides without melting. In doing so, it was heated strongly in the core, but less strongly on the outside. H3 chondrites would then originate from the surface of the mother's body, while the H4, H5 and H6 chondrites would originate from increasingly greater depths.

See also

Web links

Individual evidence

  1. ↑ Chondrite. In: Mineralienatlas Lexikon. Stefan Schorn et al., Accessed on August 4, 2018 .
  2. ^ F. Heide, F. Wlotzka, Kleine Meteoritenkunde, 3rd edition, Springer-Verlag 1988.