  Radiative transfer modelling of AGB circumstellar envelopes is applied to a
sample of AGB stars previously observed with the mid-IR imaging camera TIRCAM
(Busso et al. 1996: Paper I). We present the results of our simulations, aimed
at deriving the physical parameters of the envelope, such as the optical depth
and the radial thermal structure, the mass loss and the dust-to-gas mass ratio.
The chemical composition of the dust in the observed envelopes is discussed.
The ability of different sets of dust opacities to fit the mid-infrared spectra
is evaluated. The hypothesis of dust grain aging and annealing in O-rich
envelopes is considered in order to explain an apparent inadequacy of the
availabe opacities to describe the variety of observed spectra, as previously
noted by other authors. Various possible origins of the discrepancies are
discussed, together with their consequences on the dust grain formation
processes.
