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Aims: We investigate the mineralogy and dust processing in the circumbinarydiscs of binary post-AGB stars using high-resolution TIMMI2 and SPITZERinfrared spectra. Methods: We perform a full spectral fitting to the infraredspectra using the most recent opacities of amorphous and crystalline dustspecies. This allows for the identification of the carriers of the differentemission bands. Our fits also constrain the physical properties of differentdust species and grain sizes responsible for the observed emission features.Results: In all stars the dust is oxygen-rich: amorphous and crystallinesilicate dust species prevail and no features of a carbon-rich component can befound, the exception being EPLyr, where a mixed chemistry of both oxygen- andcarbon-rich species is found. Our full spectral fitting indicates a high degreeof dust grain processing. The mineralogy of our sample stars shows that thedust is constituted of irregularly shaped and relatively large grains, withtypical grain sizes larger than 2 micron. The spectra of nearly all stars showa high degree of crystallinity, where magnesium-rich end members of olivine andpyroxene silicates dominate. Other dust features of e.g. silica or alumina arenot present at detectable levels. Temperature estimates from our fittingroutine show that a significant fraction of grains must be cool, significantlycooler than the glass temperature. This shows that radial mixing is veryefficient is these discs and/or indicates different thermal conditions at grainformation. Our results show that strong grain processing is not limited toyoung stellar objects and that the physical processes occurring in the discsare very similar to those in protoplanetary discs.

SPITZER survey of dust grain processing in stable discs around binary post-AGB stars