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Silicon carbide within presolar
graphite.
Croat T. K. and Stadermann F. J. (2006) Lunar
Planet. Sci. XXXVII, Abstract #2048.
ABSTRACT
Most presolar silicon carbides (SiCs) are believed to
form in the circumstellar environment of asymptotic giant
branch stars (AGB) stars, since the distribution of carbon
isotopic ratios in mainstream SiCs matches well with the
ratios derived from observations of AGB stars. Recent work
on highdensity Murchison KFC graphites suggests many of
these also have an AGB origin as significant sprocess
enrichments, which are only expected in AGB stars, are found
in their internal carbides. However, unlike the SiCs and AGB
stars themselves, most of these graphites are
12C-rich
(100<12C/13C<400), suggesting
low metallicity sources. Composite grains (e.g. SiCs found
within graphite or graphite found within SiC) are of
particular interest, as they are a clear case in which both
phases form in nearby environments. Two such composite
grains have been previously reported, a graphite with an
internal SiC and a SiC with possible graphite sub-grains.
Extensive studies of KFC graphites have shown that SiCs
within graphite are rare (found in ~1 of every 1000 graphite
slices). Here we report results of transmission electron
microscopy (TEM) and NanoSIMS investigations of a single
onion-type graphite spherule that contained many internal
SiCs.
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