Mineralogical study of CI carbonaceous chondrite matrix by using electron microscopes

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1. We have studied three new CI carbonaceous chondrites(Yamato82162, Yamato-86720, Belgica-7904)from Antarctica by using electron microscopes. Our study reveals that they have many mineralogical features different from non-Antarctic CI chondrites. Two of them are mineralogically more similar to CM chondrites. All of the three chondrites apparently have undergone thermal metamorphism at temperatures 500-600'C Such significant thermal effects have not been known from other CI and CM chiondrites. These results indicate that the new CI chondrites have experienced considerably different formation histories from those for ordinary CI and CM chondrites, and suggest that there was an event in which some CI and CM parent bodies have been heated to significantly high temperatures.2. We have found fracture-filling veins of phyllosilicates in one of the new CI chondrites(Y82162). The finding provides new evidence that the phyllosilicates in CI chondrites were formed by the activity of aqueous solutions on the meteorite parent body, and that phyllosilicate formation overlapped the period of impact brecciation.3. CV type chondrites have been generally considered to have not been affected by aqueous alteration after accretion. However, we have found considerable amounts of phyllosilicates in the Mokoia CV chondrite. Evidence was found that the phyllosilicates in Mokoia were formed by aqueous alteration of olivine and pyroxene. The mechanism to form saponite from olivine is not found from terrestrial. analogues and suggests that the alteration occurred by reaction with the solar nebula in an oxidizing condition.