  We study whether the condensation of silicate dust in Mira envelopes could be
caused by cluster formation by the abundant SiO molecules. For a simplified
model of the pulsational motions of matter in the the outer layers of a Mira
variable which is guided by a numerical model for Mira pulsations, the
equations of dust nucleation and growth are solved in the co-moving frame of a
fixed mass element. It is assumed that seed particles form by clustering of SiO
molecules. The calculation of the nucleation rate is based on the experimental
data of Nuth and Donn (1982). The quantity of dust formed is calculated by a
moment method and the calculation of radiation pressure on the dusty gas is
based on a dirty silicate model. Dust nucleation occurs in the model at the
upper culmination of the trajectory of a gas parcel where it stays for a
considerable time at low temperatures while subsequent dust growth occurs
during the descending part of the motion and continues after the next shock
reversed motion. It is found that sufficient dust forms that radiation pressure
exceeds gravitational pull of the stars such that the mass element is finally
driven out of the star. Nucleation of dust particles by clustering of the
abundant SiO molecules could be the mechanism that triggers silicate dust
formation in Miras.
