SILICON-CARBIDE AND THE 11.3-MU-M FEATURE

We have synthesized crystalline SIC powders and obtained their IR exti nction spectra when embedded in KBr and CsI matrices. These spectra ar e not compatible with purely spherical grains. By contrast, they quant itatively support the assumptions that 1) the bulk optical constants o f the grains are identical with those of thin films, and very nearly i ndependent of crystal polytype, as measured by Spitzer et al. (1959) a nd 2) the granulometry is a continuous distribution of ellipsoids. All of this also applies to previously published laboratory SIC spectra. Within this framework, we can predict the spectra of the same powders if freely suspended in vacuum. Finally, using linear combination of th ese, it is possible to closely fit the various spectral shapes exhibit ed by the IRAS/LRS data for C-rich stars.