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Fayalite-silica association in unequilibrated ordinary chondrites: Evidence for aqueous alteration on a parent bodyWe report ten occurrences of high-fayalite (Fa56-99 mol%; four with Fa greater than 82 mol%) olivine in association with silica in type-3 ordinary chondrites. Pyroxene with high Fs contents is much less common; Fs contents do not exceed 66 mol%, and most maxima are less than 50 mol%. In those cases where the amount of fayalite is minor relative to that of silica, the fayalite forms a layer on the silica, and shows textural evidence of formation by reaction of silica with oxidized Fe; the latter seems to have resulted from reaction of metallic Fe-Ni with an oxidant, most likely H2O vapor. The fayalite is generally in contact with pyroxene (and, in one case, olivine) having a much lower Fe/(Fe + Mg) ratios, indicating that lattice diffusion has been minimal. Formation of fayalite from SiO2 explains the low Mg content of this olivine; the Mg was sequestered inside the lattices of mafic minerals and was thus inaccessible. In contrast, the moderately high Mn contents of the fayalite indicate that an appreciable fraction of the Mn in the precursor assemblage was accessible; it was probably sited in the matrix in the form of tiny, poorly crystallized oxide grains produced by nebular condensation at temperatures too low to permit diffusion into forsterite or enstatite. The reaction of SiO2 with FeO produced by oxidation (during metamorphism) of Fe-Ni can also account for fayalitic olivine associated with SiO2 microspherules in the fine-grained matrices of type-3 ordinary chondrites and, because matrix is SiO2 normative, for other occurrences of fayalite in matrix. The presence of Mn in the fayalitic rims on the olivine of carbonaceous chondrites does not require a nebular sign.
Document ID
19950035503
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Wasson, John T.
(Univ. of California, Los Angeles, CA United States)
Krot, Alexander N
(Univ. of California, Los Angeles, CA United States)
Date Acquired
August 16, 2013
Publication Date
April 1, 1994
Publication Information
Publication: Earth and Planetary Science Letters
Volume: 122
Issue: 4-Mar
ISSN: 0012-821X
Subject Category
Lunar And Planetary Exploration
Accession Number
95A67102
Funding Number(s)
CONTRACT_GRANT: NAGW-2243
Distribution Limits
Public
Copyright
Other

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