Structural and isotopic microanalysis of presolar SiC from supernovae

Details Structural and isotopic microanalysis of presolar SiC from supernovae Structural and isotopic microanalysis of presolar SiC from supernovae

Apr 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010m%26ps...45..596m&link_type=abstract

Meteoritics and Planetary Science, Volume 45, Issue 4, pp. 596-614.

Computer Science

4

Scientific paper

We report on the microstructure, crystallography, chemistry, and isotopic compositions of seven SiC X grains and two mainstream grains from the Murchison meteorite. TEM crystallographic analysis revealed that the X grains (approximately 3μm) are composed of many small crystals (24-457nm), while the similarly sized mainstream grains are composed of only a few crystals (0.5-1.7μm). The difference in crystal size likely results from differences in their formation environments: the X grain crystals evidently formed under conditions of greater supersaturation and rapid growth compared to their mainstream counterparts. However, the same polytypes are observed in both mainstream and X grains. Six X grains and both mainstream grains are entirely the 3C-SiC polytype and one X grain is an intergrowth of the 3C-SiC and 2H-SiC polytypes. EDXS measurements indicate relatively high Mg content in the X grains (<~5 atomic%), while Mg was undetectable in the mainstream grains. The high Mg content is probably from the decay of 26Al into 26Mg. Estimates of the 26Al/27Al ratios, which range from 0.44-0.67, were made from elemental Mg/Al ratios. This range is consistent with the 26Al/27Al ratios inferred from previous isotopic measurements of X grains. We also report the first direct observations of subgrains in X grains, including the first silicides [(Fe,Ni)nSim]. Diffraction data do not match any previously observed presolar phases, but are a good fit to silicides, which are predicted stable SN condensates. Eight subgrains with highly variable Ni/Fe ratios (0.12-1.60) were observed in two X grains.

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