Physics
Scientific paper
Oct 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996m%26ps...31..835c&link_type=abstract
Meteoritics, vol. 31, pages 835-848.
Physics
14
Scientific paper
Analytical TEM observations reveal that ilmenite grains sampled from the sub-10 5m size fraction of Apollo 11 (10084) and Apollo 16 (61221, 67701) soils have rims 10-300 nm thick that are chemically and microstructurally distinct from the host ilmenite. The rims have a thin outer sublayer 10-50 nm thick that contains the ilmenite-incompatible elements Si, Al, Ca and S. This overlies a relatively thicker (50-250 nm) inner sublayer of nanocrystalline Ti-oxide precipitates in a matrix of single-crystal ilmenite that is structurally continuous with the underlying host grain. Microstructural information, as well as data from X-ray EDS and EELS analysis of the inner sublayer, suggest that both the inner and outer sub- layer assemblages are reduced, and that the inner layer is depleted in Fe, relative to the underlying ilmenite. The chemistry of the outer sublayer suggests that it is a surface deposit of sputtered or impact-vaporized components from the bulk lunar soil. The inner sublayer is part of the original host grain that has been physically and chemically processed, but not amorphized, by solar ion irradiation and possibly some subsolidus heating. The fact that the deposited outer sublayer is consistently much thinner than the radiation-altered inner sublayer indicates that only a minor fraction of the total rim volume is a product of vapor or sputter deposition. This finding is in contrast to recent descriptions of thick deposited layers on 1/3 of regolith silicate grains, and indicates that ilmenite and silicate rims as a group are different in the fraction of deposited material that they contain.
Christoffersen Roy
Keller Lindsay P.
McKay David S.
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