  We find that the radiative diffusivity associated with the radiative
viscosity generates a mixing that diminishes the efficiency of the
gravitational settling in the external layers of stellar models corresponding
to types earlier than G. The surface abundances of chemical species predicted
by the models are successfully compared with the abundances determined in
members of the Hyades open cluster. Our modeling depends on an efficiency
parameter, which is evaluated to a value close to unity, that we calibrate in
this study.
