  Reactions of SiO molecules have been postulated to initiate efficient
formation of silicate dust particles in outflows around dying (AGB) stars. Both
OH radicals and H$_2$O molecules can be present in these environments and their
reactions with SiO and the smallest SiO cluster, Si$_2$O$_2$, affect the
efficiency of eventual dust formation. Rate coefficients of gas-phase oxidation
and clustering reactions of SiO, Si$_2$O$_2$ and Si$_2$O$_3$ have been
calculated using master equation calculations based on density functional
theory calculations. The calculations show that the reactions involving OH are
fast. Reactions involving H$_2$O are not efficient routes to oxidation but may
under the right conditions lead to hydroxylated species. The reaction of
Si$_2$O$_2$ with H$_2$O, which has been suggested as efficient producing
Si$_2$O$_3$, is therefore not as efficient as previously thought. If H$_2$O
molecules dissociate to form OH radicals, oxidation of SiO and dust formation
could be accelerated. Kinetics simulations of oxygen-rich circumstellar
environments using our proposed reaction scheme suggest that under typical
conditions only small amounts of SiO$_2$ and Si$_2$O$_2$ are formed and that
most of the silicon remains as molecular SiO.
