Towards the identification of circumstellar hibonite

We have performed IR reflectance measurements on crystalline CaAl12O19 (hibonite) and derived optical constants for both principal directions of the electric field vector relative to the crystallographic c-axis. Based on the resulting dielectric functions, we calculated small particle spectra and found six maxima of the absorption cross section for spherical dust grains at the wavelengths 12.3, 15.9, 21.1, 25.1, 34.4, and 78.7 mum. For nonspherical particles, strong shifting and broadening of the two strongest resonances at 12.3 and 15.9 mum occurs. The signal-to-noise-ratio of ISO spectra of oxygen-rich AGB stars is not sufficient for a definite identification of these bands, even though there is some weak indication of their presence. We propose additional observations to clarify this point.