Coordinated EDX and micro-Raman analysis of presolar silicon carbide: A novel, nondestructive method to identify rare subgroup SiC

1Nan Liu,2Andrew Steele,1Larry R. Nittler,3Rhonda M. Stroud,3Bradley T. De Gregorio,1Conel M. O’D. Alexander,1Jianhua Wang
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12954]
1Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC, 20015, USA
2Geophysical Laboratory, Carnegie Institution for Science, Washington, DC, USA
3Materials Science and Technology Division, Code 6366, US Naval Research Laboratory, Washington, DC, USA
Published by arrangement with John Wiley & Sons

We report the development of a novel method to nondestructively identify presolar silicon carbide (SiC) grains with high initial 26Al/27Al ratios (>0.01) and extreme 13C-enrichments (12C/13C ≤ 10) by backscattered electron-energy dispersive X-ray (EDX) and micro-Raman analyses. Our survey of a large number of presolar SiC demonstrates that (1) ~80% of core-collapse supernova and putative nova SiC can be identified by quantitative EDX and Raman analyses with >70% confidence; (2) ~90% of presolar SiC are predominantly 3C-SiC, as indicated by their Raman transverse optical (TO) peak position and width; (3) presolar 3C-SiC with 12C/13C ≤ 10 show lower Raman TO phonon frequencies compared to mainstream 3C-SiC. The downward shifted phonon frequencies of the 13C-enriched SiC with concomitant peak broadening are a natural consequence of isotope substitution. 13C-enriched SiC can therefore be identified by micro-Raman analysis; (4) larger shifts in the Raman TO peak position and width indicate deviations from the ideal 3C structure, including rare polytypes. Coordinated transmission electron microscopy analysis of one X and one mainstream SiC grain found them to be of 6H and 15R polytypes, respectively; (5) our correlated Raman and NanoSIMS study of mainstream SiC shows that high nitrogen content is a dominant factor in causing mainstream SiC Raman peak broadening without significant peak shifts; and (6) we found that the SiC condensation conditions in different stellar sites are astonishingly similar, except for X grains, which often condensed more rapidly and at higher atmospheric densities and temperatures, resulting in a higher fraction of grains with much downward shifted and broadened Raman TO peaks.

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