The SiC problem: Astronomical and meteoritic evidence

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.