An overview of the research consortium,"Antarctic carbonaceouschondrites with CI affinities, Yamato-86720, Yamato-82162, andBelgica-7904

Three Antarctic meteorites, Yamato (Y)-82162,Y-86720,and Belgica (B)-7904,are unusual carbonaceous chondrites having characteristics of both CI and CM chondrite groups. Therefore, a research consortium has been carried out in 1987-1990,and all the results are summarized here. Remarkable results are; (1) Several minerals that are rare or new in carbonaceous chondrites were found in the consortium chondrites. They are sodian talc, FeO-bearing periclase or magnesiowustite, an unknown Fe-Ca-O-S mineral, rhodochrosite, chalcopyrite, eskolaite, schreibersite, Co-rich metal, Ni-Co-free kamacite, and so on. (2) Phyllosilicate veins were firstly found in Y-82162,suggesting that the hydration took place in the parent body. (3) Detailed mineralogy and petrology of altered chondrules and clasts suggest that most of the phyllosilicates in chondrules and clasts were produced from anhydrous precursors in the solar nebula. (4) At the final stage of the chondrite formation, the three chondrites underwent intense impact-shock or metamorphic heating after the hydrous alteration on their parent body. (5) Mineralogical, chemical, and oxygen isotopic data indicate that the three chondrites have some characteristics of both CI and CM groups, and the genetical relationships between the two groups and the consortium chondrites have been suggested. In addition, a new carbonaceous chondrite group CY is proposed for the three unusual chondrites.