Carbonaceous chondrites CR


CR Chondrites

Synonyms: Renazzo-like carbonaceous chondrites


General: The chondrites of the CR group are named for their type specimen Renazzo, a meteorite that fell in Emilia-Romagna, Italy, in 1824. Renazzo and a few related finds have been initially classified as "type II" CM chondrites. However, the CR chondrites are very different from the chondrites of the CM group. They are more closely related to two other groups of metal-rich carbonaceous chondrites, the CH and CB chondrites, and together they form the CR clan.


Description: CR chondrites are easily distinguished from most other carbonaceous chondrites due to the fact that they contain abundant free metal in the form of nickel-iron and iron sulfide of up to 10%. This metal is found in the black matrix as well as in the large and clearly visible chondrules that make up about 50% of the meteorites. Sometimes these orange-coloured chondrules are "armored", that means they are imbedded into small rims of nickel-iron or iron sulfide, and sometimes the metal even appears as rounded grains within the matrix or near to chondrule surfaces. The chondrules are usually well defined, and nearly all CR chondrites belong to petrologic type 2.


Mineralogy: Like the members of theCM group, the CR chondrites were subjected to the process of aqueous alteration. They contain hydrosilicates, traces of water, and magnetite. Reduced nickel-iron metal and iron sulfides are found in the black matrix as well as in the chondrules.


Origin and Formation: Scientists have searched for the origin of the members of the CR group, comparing different reflectance spectra of asteroids with the spectra of the known CR members. There is quite a good match between the spectra of the CR members and one of the most prominent asteroids in our solar system, 2 Pallas, the second largest asteroid known. In addition, maybe we have other samples of this prospective parent body since modern research suggests that two other groups of carbonaceous chondrites are closely related to the CR chondrites - the CH chondrites and CB chondrites. Together they form the so-called CR clan that either has its origin in one and the same parent body or at least in a common region of the early solar nebula in which they formed under similar, more reducing conditions.


Members: If we exclude all probable pairings, especially from Nortwest Africa and the blue-ice fields of Antarctica, there are only about 25 different CR chondrites known. The most prominent members are the two witnessed falls, Al Rais and Renazzo, but also the less prominent desert finds are highly coveted among collectors, such as DaG 574, El Djouf 001, or the more common NWA 801. Our team also found an ultra-rare metal-rich carbonaceous chondrite that was first thought to be the only primordial CR3 chondrite recovered, so far. However, subsequent analysis, and isotope testing have shown that Sahara 00182 has to be considered to be an ungrouped carbonaceous chondrite with some affinities to the CR clan.

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