Phyllosilicates in the matrix of the unique carbonaceous chondrite, LEW 85332 and possible implications for the aqueous alteration of CI chondrites

Details Phyllosilicates in the matrix of the unique carbonaceous chondrite, LEW 85332 and possible implications for the aqueous alteration of CI chondrites Phyllosilicates in the matrix of the unique carbonaceous chondrite, LEW 85332 and possible implications for the aqueous alteration of CI chondrites

May 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..377b&link_type=abstract

Meteoritics, vol. 32, pages 377-388

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11

Scientific paper

The fine-grained matrix of the unique, unequilibrated carbonaceous chondrite, LEW 85332 has been studied by SEM, electron microprobe analysis and TEM. Compositionally, LEW 85332 has a matrix which is more Fe-rich than typical CI chondrites but has elemental abundance ratios which appear to be closer to CI matrices than to CM or CR chondrites. The mineralogy of the matrix is dominated by phyllosilicate phases, predominantly interlayered Fe-rich serpentine/saponite; anhydrous silicate phases such as olivine and pyroxene are rare. Minor magnetite, troilite and ferrihydrite also occur associated with the phyllosilicates. Despite the high degree of weathering in LEW 85332, the phyllosilicates appear to have an extraterrestrial origin, but the ferrihydrite may have formed as a result of terrestrial weathering of Fe,Ni metal. The compositional and mineralogical data suggest that matrix of LEW 85332 may represent a very early stage in the type of aqueous alteration experienced by the CI chondrites, although it is improbable that LEW 85332 was a precursor to the CI chondrites, because of its high abundance of chondrules. The absence of carbonates, the high Fe content of the matrix and phyllosilicate phases and relatively low abundance of magnetite all indicate that the degree of oxidation and leaching of LEW 85332 matrix was significantly less than that experienced by the CI chondrites. The absence of clear evidence for alteration of chondrules suggests that either the formation of the hydrous phases in the matrix occurred prior to accretion or that alteration occurred on a parent body and involved limited amounts of fluid, such that the reactions took place preferentially and exclusively within the fine-grained (anhydrous?) matrix materials.

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