  The winds observed around AGB stars are generally attributed to radiation
pressure on dust formed in the dynamical atmospheres of these long-period
variables. The composition of wind-driving grains is affected by a feedback
between their optical properties and the resulting heating due to stellar
radiation. We explore the gradual Fe enrichment of wind-driving silicate grains
in M-type AGB stars to derive typical values for Fe/Mg and test the effects on
wind properties and synthetic spectra. We present new DARWIN models that allow
for the growth of silicate grains with a variable Fe/Mg ratio and predict
mass-loss rates, wind velocities, and grain properties. Synthetic spectra and
other observables are computed with the COMA code. The self-regulating feedback
between grain composition and radiative heating, in combination with quickly
falling densities in the stellar wind, leads to low values of Fe/Mg, typically
a few percent. Nevertheless, the models show distinct silicate features around
10 and 18 microns. Fe enrichment affects visual and near-IR photometry
moderately, and the new DARWIN models agree well with observations in (J-K) vs.
(V-K) and Spitzer color-color diagrams. The enrichment of the silicate dust
with Fe is a secondary process, taking place in the stellar wind on the surface
of large Fe-free grains that have initiated the outflow. Therefore, the
mass-loss rates are basically unaffected, while the wind velocities tend to be
slightly higher than in corresponding models with Fe-free silicate dust. The
gradual Fe enrichment of silicate grains in the inner wind region should
produce signatures observable in mid-IR spectro-interferometrical measurements.
Mass-loss rates derived from existing DARWIN models, based on Fe-free
silicates, can be applied to stellar evolution models since the mass-loss rates
are not significantly affected by the inclusion of Fe in the silicate grains.
