  Main ions influence impurity dynamics through a variety of mechanisms; in
particular, via impurity-ion collisions. To lowest order in an expansion in the
main ion mass over the impurity mass, the impurity-ion collision operator only
depends on the component of the main ion distribution that is odd in the
parallel velocity. These lowest order terms give the parallel friction of the
impurities with the main ions, which is typically assumed to be the main cause
of collisional impurity transport. Next-order terms in the mass ratio expansion
of the impurity-ion collision operator, proportional to the component of the
main ion distribution that is even in the parallel velocity, are usually
neglected. However, in stellarators, the even component of the main ion
distribution can be very large. In this article, such next-order terms in the
mass ratio expansion of the impurity-ion collision operator are retained, and
analytical expressions for the neoclassical radial flux of trace impurities are
calculated in the Pfirsch-Schl\"uter, plateau and $1/\nu$ regimes. The new
terms provide a drive for impurity transport that is physically very different
from parallel friction: they are associated to anisotropy in the pressure of
the main ions, which translates into impurity pressure anisotropy. It is argued
that main ion pressure anisotropy must be taken into account for a correct
description of impurity transport in certain realistic stellarator plasmas.
Examples are given by numerically evaluating the analytical expressions for the
impurity flux.
