Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.252l&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 252
Mathematics
Logic
Scientific paper
Magnetite occurs in CI and CM chondrites as plaquettes, framboids, and radially structured spherolites (Jedwab, 1967, 1971). Recent ion microprobe investigations of the O-isotopic compositions of individual magnetite grains in Orgueil, Alais, Ivuna, and Revelstoke revealed enormous variations in delta^18O (-25 to +25) in the same meteorite (Hyman et al., (1991)). A planetary process involving water should lead to formation of grains of similar O-isotopic composition. Individual magnetites in Alais (CI1), Orgueil (CI1), Y-82162 (CI2), Essebi (CM2) and Acfer-207 have been studied in detail by us (e.g., more than 300 electron microprobe analyses). The fact that all morphological types occur together in clusters make an in situ formation not very probable. The stacking morphology of the plaquettes is in fact the strongest argument against epitaxial growth of magnetite along the rhombohedral surfaces of carbonates as suggested by Kerridge et al. (1979). The stacks occur as rounded or oval discs with the top and bottom discs displaying several oblique crystal surfaces and one perpendicular to the axis of the disc stacking. Epitaxial growth should--in contrast to the encountered plaquette morphology--produce three sets of stacks every one parallel to two opposing surfaces of the rhombohedral faces with diminishing disc sizes towards the centre of the rhombohedron. This epitaxial growth would also produce plaquettes with rhombohedral outlines and not circular or oval ones. In none of the studied meteorites were three directional rhombohedral magnetite plaquettes encountered. In Alais a sequence of formation: Magnetite--pyrrhotite--carbonate was found. The magnetites in all C-chondrites studied, are pure Fe3O4 regardless of their morphology and the assemblage in which they occur. Only the magnetites in Y-82162 are enriched in MgO and some of them in MnO. The present investigations, variability of the chemical composition of magnetite in Y-82162 and the oxygen isotopic results of Hyman et al. (1991) cast strong doubt on the meteorite parent body models for the formation of magnetite in C- chondrites proposed by Kerridge et al. (1979). REFERENCES Jedwab J. (1967) Earth and Planet. Sci. Letters 2, 440-444. Jedwab J. (1971) Icarus 15, 319-340. Hyman M., Zinner E.K. and Rowe M.W. (1991) Meteoritics (abstract) 26, 350. Kerridge J.F., Mackay A.L. and Boynton W.V. (1979) Science 205, 395-397.
El Goresy Ahmed
Löhn B.
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