The oool extended shells of giants and supergiants are well known to be places of copious dust formation as indicated by the occurrence of pronounced extinction, reddening, and polarization of the continuous star light and by the appearance of particular absorption features, both manifesting the interaction of photons with particles considerably larger than atoms or molecules.
The accepted explanation of these phenomena is the formation of circumstellar dust, i.e. small solid particles with a typical size of the order of 0.1 micron. However, the analysis of these effects yields only information on the interaction of photons with certain functional groups within the clusters - e.g. Si-O, C-H, C-C bending and Si-O, C-C,... stretching vibrations - and thus allows no definite determination of the “real” physical structure and chemical composition of the grain particles. Therefore, observational conclusions concerning the properties of circumstellar dust can provide only some “mean” information which allows no definitive conclusions regarding the true nature of the observed grains (geometrical shape, crystalline structure,...?).