TEM analyses of unusual presolar silicon carbide: Insights into the
  range of circumstellar dust condensation conditions
  Presolar silicon carbide (SiC) grains in meteoritic samples can help
constrain circumstellar condensation processes and conditions in C-rich stars
and core-collapse supernovae. This study presents our findings on eight
presolar SiC grains from AGB stars (four mainstream and one Y grain) and
core-collapse supernovae (three X grains), chosen on the basis of {\mu}-Raman
spectral features that were indicative of their having unusual non-3C polytypes
and/or high degrees of crystal disorder. Analytical transmission electron
microscopy (TEM), which provides elemental compositional and structural
information, shows evidence for complex histories for the grains. Our TEM
results confirm the presence of non-3C,2H crystal domains. Minor element
heterogeneities and/or subgrains were observed in all grains analyzed for their
compositions. The C/O ratios inferred for the parent stars varied from 0.98 to
greater than or equal to 1.03. Our data show that SiC condensation can occur
under a wide range of conditions, in which environmental factors other than
temperature (e.g., pressure, gas composition, heterogeneous nucleation on
pre-condensed phases) play a significant role. Based on previous {\mu}-Raman
studies, about 10% of SiC grains may have infrared (IR) spectral features that
are influenced by crystal defects, porosity, and/or subgrains. Future
sub-diffraction limited IR measurements of complex SiC grains might shed
further light on the relative contributions of each of these features to the
shape and position of the characteristic IR 11-{\mu}m SiC feature and thus
improve the interpretation of IR spectra of AGB stars like those that produced
the presolar SiC grains.
