Spectroscopic and TEM studies of phyllosilicates in carbonaceous chondrite

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CM chondrites experienced aqueous alteration. Due to the aqueous alteration, mesostses in chondrules were replaced by phyllosilicates. Such phyllosilicates are various in colors, for example, green, yellowish green, yellowish brown, reddish brown, and so on. In this study, phylloslicates in Murchison, Murray, Y75293, Y791 198, Mighei, Nogoya, and Y82042 were investigated by visible light spectroscopy and electron microprobe analysis. Colors of such phyllosilicates were compared quantitatively by using L*a*b* color system. Compared with compositional variation of the phyllosilicates among differently altered CM chondrites, changes in colors in the phyllosilicates are complex. Complexity of changes in color depends on both the differences in minerals which suffered alteration and redox states during phyllosilicate formation. Differences in colors in phyllosilicates are due to broad absorption band around 700 nm (probably due to IVCT between Fe^<3+> and Fe^<2+>) and a shoulder around 450-500 nm (probably due to six coordinated Fe^<3+> cations). It is expected that green phyllosilicates and reddish brown ones have different Fe^<3+>/Fe^<2+> ratios. We are now seeking a convenient method to acquire Fe^<3+>/Fe^<2+> ratios in serpentine-group minerals.