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We study the grain properties and location of the forsterite crystals in thecircumstellar environment of the pre-planetary nebula (PPN) IRAS 17150-3224 inorder to learn more about the as yet poorly understood evolutionary phase priorto the PPN. We use the best-fit model for IRAS 17150-3224 of Meixner et al.(2002) and add forsterite to this model. We investigate different spatialdistributions and grain sizes of the forsterite crystals in the circumstellarenvironment. We compare the spectral bands of forsterite in the mid-infraredand at 69 micrometre in radiative transport models to those in ISO-SWS andHerschel/PACS observations. We can reproduce the non-detection of themid-infrared bands and the detection of the 69 micrometre feature with modelswhere the forsterite is distributed in the whole outflow, in the superwindregion, or in the AGB-wind region emitted previous to the superwind, but wecannot discriminate between these three models. To reproduce the observedspectral bands with these three models, the forsterite crystals need to bedominated by a grain size population of 2 micrometre up to 6 micrometre. Wehypothesise that the large forsterite crystals were formed after the superwindphase of IRAS 17150-3224, where the star developed an as yet unknown hyperwindwith an extremely high mass-loss rate (10^-3 Msol/yr). The high densities ofsuch a hyperwind could be responsible for the efficient grain growth of bothamorphous and crystalline dust in the outflow. Several mechanisms are discussedthat might explain the lower-limit of 2 micrometre found for the forsteritegrains, but none are satisfactory. Among the mechanisms explored is a possibleselection effect due to radiation pressure based on photon scattering onmicron-sized grains.

Micron-sized forsterite grains in the pre-planetary nebula of IRAS 17150-3224 - Searching for clues on the mysterious evolution of massive AGB stars