The SiC problem: astronomical and meteoritic evidence.
SPECK A.K., HOFMEISTER A.M. and BARLOW M.J.
Abstract (from CDS):
Presolar grains of silicon carbide, found in meteorites and interpreted as having had an origin around carbon stars from their isotopic composition, have all been found to be of the β-SiC polytype. Yet, to date, fits to the 11.3 µm SiC emission band of carbon stars had been obtained only for α-SiC grains. We present thin-film infrared (IR) absorption spectra that were measured in a diamond-anvil cell for both the α- and β-polymorphs of synthetic SiC, and we compare the results with previously published spectra that were taken using the KBr matrix method. We find that our thin-film spectra have positions nearly identical to those obtained previously from finely ground samples in KBr. Hence, we show that this discrepancy has arisen from inappropriate ``KBr corrections'' having been made to laboratory spectra of SiC particles dispersed in KBr matrices. We refitted a sample of carbon star mid-IR spectra, using laboratory data with no KBr correction applied, and show that β-SiC grains fitted the observations while α-SiC grains did not. The discrepancy between meteoritic and astronomical identifications of the SiC type is therefore removed. This work shows that the diamond-anvil cell, thin-film method can be used to produce mineral spectra that are applicable to cosmic environments without further manipulation.