Chondrite

Chondrites form with a share of about 86 percent, the largest class of meteorites. Your name is due to trapped small silicate spheres, called chondrules, which are embedded in a fine-grained groundmass. The mineralogical composition of the chondrites is dominated by the minerals olivine, pyroxene and plagioclase. However, they also contain (with a few exceptions in the carbonaceous chondrites ) are always metallic nickel iron ( see Figure chondrite Holbrook ) and iron sulfide ( troilite ).

Chondrites can be regarded as cosmic sedimentary rocks. Often they are also called undifferentiated meteorites, since their chemical composition, with the exception of gaseous and volatile elements such as sodium and potassium, or noble gases, the composition of the photosphere of our sun, and thus that of the original solar nebula corresponds. Age determinations by measurements of radioactive isotopes have shown that chondrites have already emerged in the early days of the solar system 4.5 billion years ago.

History

The Chondritenfall of Lucé in France in 1768 was investigated by the French chemist Antoine Laurent Lavoisier. At that time, however, the existence of meteorites was not generally accepted, and Lavoisier explained the stone for a kind of iron pyrites. The eye-catching chondrules that have been observed in rocks from a variety of places of discovery, however, eventually led to recognize their extraterrestrial origin. The famous French chemist Louis Pasteur examined in 1864 also a meteorite, which was assigned to the carbonaceous chondrites.

Classification

Chondrites can be divided into subclasses again. After frequency in the first place are the ordinary chondrites. The carbonaceous chondrites are so named because they contain large amounts of carbon (also known as organic compounds). Your fracture surface therefore looks more or less as black as coal. In addition to chondrules are found in their fine-grained matrix also presolar minerals and calcium - aluminum - rich inclusions.

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

• Ordinary Chondrites. You are using the total iron content and the iron content of nickel divided into: H- chondrites ( for high iron ), with 22-30 % and 17-23 % total iron metal,
• L- chondrites ( for low iron ), with 20-24 % total iron and 4-9 % metal
• L L- chondrites ( for low iron, low metal), with 19-22 % total iron and 0.3-3 % metal.

In the practical determination of the class of Eisenoxigehalt in olivine is used, it is inversely proportional to the total iron and is: For H- chondrite 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 type 3 chondrites unequilibrierten (see below), the Fa content is variable between 0 and 50% Fa

• Enstatite chondrites. They contain no olivine and pyroxene ( here enstatite ) FS - content ( mol% ferrosilite or Fe / (Fe Mg) ) is less than 1 EH- chondrites
• EL chondrites

• CI ( Ivuna group)
• CO ( Ornans group)
• CV ( Vigarano group)
• CM ( Mighei group)
• CK ( Karoonda group)
• CR ( Renazzo group)
• CH ( Metal Empire)
• CB ( Bencubbinite )

A survey of the frequency of the elements in the various Chondritenklassen can at Kallemyn et al. ( " Geochemistry of ordinary chondrites ", Geochimica et Cosmochimica Acta, 1989, 2747 ) are found.

In addition to the chemical classification are chondrites according to a scheme that has been proposed by scientists Van blarney and Wood 1967, divided into the petrologic types 1-6, sometimes 1-7. Chondrites of petrologic type 3 are " unequilibriert " because they represent material that was virtually unchanged by accumulation from the solar nebula after the formation of the parent body. In contrast chondrites were metamorphosed increasingly thermally type 4-6. These chondrites are recrystallized here. As a result blur in type 6 chondrites, the boundaries between chondrules and matrix (see the image of the L6 chondrite Holbrook ). Meteorites of type 1 and 2 were not altered thermally, but have a " watery " experienced metamorphism and contain crystal water-containing silicates ( clay minerals ). While chondrites of type 3 to 6 contain a maximum of 3 percent by weight of water, type 2 chondrites can up to 18 percent by weight and type 1 chondrites contain more than 20 weight percent water. Type CI1 chondrites contain chondrules, although it is not clear whether they ever had chondrules, or whether they have been destroyed by the aqueous alteration.

In ordinary chondrites and chondrite Entstatit the petrologic types 3-7 carbonaceous chondrites in the petrologic types 1-6 (up to CK5 / 6) and come before.

Chondrites of different petrologic type may well come from the same parent body. Thus, the " onion skin model " has been proposed for the parent body of the H chondrites. Under this model, the parent body of H chondrites would be an undifferentiated asteroid, which was again heated by decay of radioactive nuclides after its formation 4.56 billion years without melting, however. He was in the core strong, but less strongly heated from the outside. H3 chondrites would then originate from the surface of the parent body, while the H4, H5 and H6 chondrites would come from increasingly greater depths.