Instituut voor Sterrenkunde, KULeuven, Celestijnenlaan 200B, B3001 - Heverlee, BELGIUM
One of the more spectacular results by ISO may have been the detection of rich spectra of crystalline silicates in the circumstellar environments of old (Waters et al. 1996) and young (Waelkens et al. 1996) stars. Before ISO crystalline silicate features had only been observed in solar-system objects and the disks of Beta Pictoris like stars. The study of the silicate spectra then appears most promising as an observational tracer of grain processing in circumstellar disks, possibly leading to planet formation. A gratifying circumstance has been that ISO was operational when the dusty and extremely bright comet Hale-Bopp appeared in the sky.
We review observational evidence of the presence of crystalline silicates and water ice in the spectra of the disks that surround young stars. Crystalline silicate features so far seem to be absent in spectra of the interstellar medium and in the disks of the youngest stellar objects. On the other hand, they do appear in the spectra of evolved objects. An attempt is made to confront these data with our understanding of the cosmic dust cycle.