Carbonaceous chondrites CO

CO Chondrites

Synonyms: Ornans-like carbonaceous chondrites


General: The chondrites of this group are named for their type specimen Ornans, a meteorite that fell in France in July 1868. The CO chondrites are closely related to the CV and Ck chondrites, and together they form a distinct clan of carbonaceous chondrites.


Description: Although the CO chondrites are closely related to the CV chondrites, they are rather distinct in structure, and overall appearance. The chondrites of the CO group are mostly of a more black appearance (although a few are dark-grey, such as the type specimen Ornans), and exhibit much smaller chondrules. These tiny chondrules are packed densely within the matrix, representing over 70% of the meteorite. In CV chondrites, this ratio is reversed; only about 30% of the meteorite is composed of large chondrules. All members of the CO group belong to the petrologic type 3.


Mineralogy: The mineralogy of CO chondrites is remarkably similar to that of the CV group. As for the CVs, the members of the CO group also contain CAIs, but these inclusions are commonly much smaller and more sparsely distributed throughout the matrix. Typical for the COs are clearly visible, small inclusions of free metal, mostly nickel-iron, that appear like tiny flakes on the polished surfaces of a fresh, unweathered CO3.


Origin and Formation: From the abundance of free metal and nickel-iron we can conclude that the CO chondrites formed under even more reducing conditions than the CV chondrites of the reduced subgroup which don't show that much metal in its free form. It seems highly improbable that both groups formed on one and the same parent body. Therefore, many researchers suggest that the CV and the CO group represent a distinct clan of carbonaceous chondrites that formed in the same region of the early solar nebula. However, the conditions under which the COs formed must have been different from the conditions under which the CVs formed because there are obvious differences. Scientists have not yet identified a spectral match for a possible parent body for the CV or the CO chondrites.


Members: There are only about 35 members of this group if we don't count all the probable pairings, especially from the Dar al Gani region, Libya, where many CO chondrites have been found. It's more than probable that all the Dar al Gani COs arrived in one or two distinct falls. Famous CO members include the witnessed falls Felix, Kainsaz, Lancé, and Ornans.


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