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Carbonaceous chondrites CK
Synonyms: Karoonda-like carbonaceous chondrites
General: The chondrites of the CK group are named for their type specimen Karoonda, a meteorite that fell in Australia in 1930. Initially, Karoonda, and a few related Antarctic finds were regarded as members of the CV group, and they were designated as CV4-5. However, more recently, they have been given their own group as they differ in some respects from all the other groups of carbonaceous chondrites. Nevertheless, together with the CV and CO chondrites they form a distinct clan.
Description: CK chondrites are dark-grey or blackish rocks with shiny black fusion crusts. Chondrules are sparse - just about 15 vol% - and a bit smaller and less defined than in CV chondrites. The members of this group belong to the petrologic types 3 - 6, although most of them have been classified as CK4. CAIs (calcium-aluminium-inclusions) are common, and often large.
Mineralogy: The dark-grey or blackish appearance of the CK chondrites is due to a high percentage of magnetite that is dispersed in a matrix of dark, sometimes shock-blackened silicates, consisting of iron-rich olivine and pyroxene. All of this indicates that they formed under oxidizing conditions, but they don't show any signs of a primary aqueous alteration or phyllosilicates.
Origin and Formation: Elemental abundances as well as the oxygen isotopic signatures suggest that the CK chondrites are closely related to the chondrites of the CO and CV groups and belong to the same clan. They plot somewhere in between those two groups which can also be seen in the size of the chondrules that are intermediate between the typical sizes of CV and CO chondrules. Most CK chondrites contain large CAIs, indicating that their formation history took place in the early hours of the primordial nebula. In addition, some CK members exhibit shock veins, or other signs of shock that indicate a violent history for their parent body, such as an impact history. However, scientists have not yet identified a spectral match for a possible parent body for these rare carbonaceous chondrites yet.
Members: There are only about 35 different CK members known if we exclude all the pairings that have been found so far in the hot deserts of Africa and in the blue-ice fields of Antarctica. Famous members of the CK group include Karoonda, and Maralinga, both having been classified as CK4, and some rare members with higher or lower petrologic grades, such as DAG 412 from Libya, a rare and famous CK5, or Dhoafr 015 from the Sultanate of Oman, an ultra-rare unequilibrated CK3.