Bencubbinites / B Chondrites / CB
Carbonaceous Chondrites / CB (bencubinites)
Synonyms: Bencubbinites, Bencubbin-like carbonaceous chondrites
General: The chondrites of this newly designated group are named for their type specimen Bencubbin, a meteorite that has been recovered in Australia in 1930. The CB chondrites are strange meteorites that contain more than 50% nickel-iron. If you consider this, they could be easily regarded as true stony-iron meteorites, but their mineralogical and chemical properties clearly put them into the class of the carbonaceous chondrites, or more strictly speaking into the CR clan.
Description: The CB chondrites can be easily distinguished from all other meteorite groups due to their high metal content, and they typically show mm-sized metal globules, and often cm-sized chondrules, with the metal globules often distorted by shock or partial melting. All CB members recovered so far belong to petrologic type 3.
Mineralogy: Besides abundant free metal, the CB members contain highly reduced silicates as well as armed chondrules similar to those found in the members of the CR group. Some members also contain CAIs (calcium-aluminium-inclusions), e.g., the meteorite HaH 237 from Libya, which was previously classified as a metal-rich CH chondrite. This shows the close relationship between the CH and CB groups, which are both members of the CR clan. The CB chondrites are further divided into the CBa subgroup, characterized by large metal globules and chondrules, and into the more metal-rich CBb subgroup with smaller chondrules and metal globules.
Origin and Formation: It is probable that the CB chondrites - like all members of the CR clan - formed under different conditions in the same region of the primordial solar nebula, but it is also possible that they are all part of one and the same parent body. In the latter case, 2 Pallas, the second largest asteroid of our solar system, would be a prominent candidate to be the original parent body of the meteorites of this clan. At least the reflectance spectra of the CR chondrites match the spectra of 2 Pallas quite closely, suggesting that those meteorites might have been derived from this large asteroid through impact events. On the other hand the meteorites of the CR clan don't show too many signs of an impact history such as brecciation, shock-veins etc. so that they might have been derived from much smaller parent bodies which aren't to be identified that easily.
Members: Currently, only eight meteorites constitute this ultra-rare group: the type specimen Bencubbin, Fountain Hills, Gujba, NWA 1814, and Weatherford constituting the CBa main group, and the more metal-rich HAH 237, QUE 94411, and the newly found Isheyevo forming the CBb subgroup. Of all these members, Gujba is the only witnessed fall, and arguably the most beautiful CB chondrite in existence.