  We discuss the dust chemistry and growth in the circumstellar envelopes
(CSEs) of Thermally Pulsing Asymptotic Giant Branch (TP-AGB) star models
computed with the COLIBRI code, at varying initial mass and metallicity
(Z=0.001, 0.008, 0.02, 0.04, 0.06). A relevant result of our analysis deals
with the silicate production in M-stars. We show that, in order to reproduce
the observed trend between terminal velocities and mass-loss rates in Galactic
M-giants, one has to significantly reduce the efficiency of chemisputtering by
H2 molecules, usually considered as the most effective dust destruction
mechanism. This indication is also in agreement with the most recent laboratory
results, which show that silicates may condense already at T=1400 K, instead
than at Tcond=1000 K, as obtained by models that include chemisputtering. From
the analysis of the total dust ejecta, we find that the total dust-to-gas
ejecta of intermediate-mass stars are much less dependent on metallicity than
usually assumed. In a broader context, our results are suitable to study the
dust enrichment of the interstellar medium provided by TP-AGB stars in both
nearby and high redshift galaxies.
