Carbonaceous chondrite

A carbonaceous chondrite or a C-type chondrite is a type of chondritic meteorite which contains high levels of water and organic compounds, representing only a small proportion (~5%) of known meteorites. Their bulk composition is mainly silicates, oxides and sulfides, while the minerals olivine and serpentine are characteristic. The presence of volatile organic chemicals and water indicates that they have not undergone significant heating (&gt;200°C) since they formed, so their composition is considered to be representative of the solar nebula from which the solar system condensed.

Carbonaceous chondrites are grouped according to distinctive compositions thought to reflect the type of parent body from which they originated. These are named after a prominent meteorite - often the first to be discovered - in the group.

Some famous carbonaceous chondrites are: Orgueil, Ivuna, Murray, Murchison, Tagish Lake and Allende.

CI group
This group, named after the Ivuna meteorite, are considered the least altered of all carbonaceous chondrites. They typically contain a high proportion of water (up to 20%), and organic matter in the form of amino acids and PAHs. Aqueous alteration promotes a composition of hydrous phyllosilicates, magnetite, and olivine crystals occurring in a black matrix, and a possible lack of chondrules. It is thought they have not been heated above 50°C, indicating that they condensed in the cooler outer portion of the solar nebula.

There are only 5 known CI chondrites: Ivuna, Orgueil, Alais, Tonk and Revelstoke.

Organic matter
Ehrenfreund et al. (2001) found that amino acids in Ivuna and Orgueil were present at much lower concentrations than in CM chondrites (~30%), and that they had a distinct composition high in β-alanine, glycine, γ-ABA, and β-ABA but low in AIB and isovaline. This implies that they had formed by a different synthetic pathway, and on a different parent body from the CM chondrites. Most of the organic carbon in CI and CM carbonaceous chondrites is an insoluble complex material. That is similar to the description for kerogen. A kerogen-like material is also in the ALH84001 Martian meteorite (an achondrite).

The CM meteorite from Murchison, Victoria has over 70 extraterrestrial amino acids and other compounds including carboxylic acids, hydroxy carboxylic acids, sulphonic and phosphonic acids, aliphatic, aromatic and polar hydrocarbons, fullerenes, heterocycles, carbonyl compounds, alcohols, amines and amides.