  Current knowledge suggests that the dust-driven wind scenario provides a
realistic framework for understanding mass loss from C-rich AGB stars. For
M-type objects, however, recent detailed models demonstrate that radiation
pressure on silicate grains is not sufficient to drive the observed winds,
contrary to previous expectations. In this paper, we suggest an alternative
mechanism for the mass-loss of M-type AGB stars, involving the formation of
both carbon and silicate grains due to non-equilibrium effects, and we study
the viability of this scenario. We model the dynamical atmospheres and winds of
AGB stars by solving the coupled system of frequency-dependent radiation
hydrodynamics and time-dependent dust formation, using a parameterized
description of non-equilibrium effects in the gas phase. This approach allows
us to assess under which circumstances it is possible to drive winds with small
amounts of carbon dust and to get silicate grains forming in these outflows at
the same time. The properties of the resulting wind models, such as mass loss
rates and outflow velocities, are well within the observed limits for M-type
AGB stars. Furthermore, according to our results, it is quite unlikely that
significant amounts of silicate grains will condense in a wind driven by a
force totally unrelated to dust formation, as the conditions in the upper
atmosphere and wind acceleration region put strong constraints on grain growth.
The proposed scenario provides a natural explanation for the observed
similarities in wind properties of M-type and C-type AGB stars and implies a
smooth transition for stars with increasing carbon abundance, from
solar-composition to C-rich AGB stars, possibly solving the long-standing
problem of the driving mechanism for stars with C/O close to one.
