Most CV chondrites contain little if any phyllosilicate mineralization. A petrographic and transmission electron microscopy study of the Mokoia CV carbonaceous chondrite shows that the matrix, chondrules, aggregates, and inclusions all contain considerable amounts of phyllosilicates. Those in the matrix are mostly Fe-bearing saponite, while those in chondrules, aggregates, and inclusions are both Febearing saponite and Na-rich phlogopite. Serpentine occurs as minor coherent intergrowths within the phlogopite. The saponite resulted from aqueous alteration of Fe-rich olivine, and the phlogopite resulted from alteration of Ca-rich clinopyroxene, anorthite, spinel, and Ca-Al-Si-rich glass. Iron-rich olivines in the Mokoia matrix contain extremely thin, planar zones parallel to (100), with diffuse streaks along a* in their diffraction patterns. Such planar zones are not known from terrestrial olivines. Saponite forms preferentially along these zones, with its (001) planes roughly parallel to olivine (100). The planar zones probably consist of precipitates of Fe oxide or Fe hydroxide that were produced during oxidation of olivine while it cooled from a high temperature. By formation of the Fe-rich precipitates, olivine became Fe deficient and thus was structurally disturbed. During subsequent hydration, saponite formed by preferentially replacing these regions in olivine. Simultaneously, part of the Fe was mobilized and precipitated as magnetite along the olivine peripheries. The mineralogy and occurrence of phyllosilicates in Mokoia differ from those in the CI and CM chondrites. The differences suggest that aqueous alteration of the three meteorite groups probably occurred under a variety of conditions.
ASJC Scopus subject areas
- Geochemistry and Petrology