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Presolar silicon carbide (SiC) grains in meteoritic samples can helpconstrain circumstellar condensation processes and conditions in C-rich starsand core-collapse supernovae. This study presents our findings on eightpresolar SiC grains from AGB stars (four mainstream and one Y grain) andcore-collapse supernovae (three X grains), chosen on the basis of {\mu}-Ramanspectral features that were indicative of their having unusual non-3C polytypesand/or high degrees of crystal disorder. Analytical transmission electronmicroscopy (TEM), which provides elemental compositional and structuralinformation, shows evidence for complex histories for the grains. Our TEMresults confirm the presence of non-3C,2H crystal domains. Minor elementheterogeneities and/or subgrains were observed in all grains analyzed for theircompositions. The C/O ratios inferred for the parent stars varied from 0.98 togreater than or equal to 1.03. Our data show that SiC condensation can occurunder a wide range of conditions, in which environmental factors other thantemperature (e.g., pressure, gas composition, heterogeneous nucleation onpre-condensed phases) play a significant role. Based on previous {\mu}-Ramanstudies, about 10% of SiC grains may have infrared (IR) spectral features thatare influenced by crystal defects, porosity, and/or subgrains. Futuresub-diffraction limited IR measurements of complex SiC grains might shedfurther light on the relative contributions of each of these features to theshape and position of the characteristic IR 11-{\mu}m SiC feature and thusimprove the interpretation of IR spectra of AGB stars like those that producedthe presolar SiC grains.

TEM analyses of unusual presolar silicon carbide: Insights into the range of circumstellar dust condensation conditions