  We analyze the stability of a dilute plasma with thermal and composition
gradients in the limit where conduction is slow compared to the dynamical
timescale. We find necessary and sufficient conditions for stability when the
background magnetic field is either parallel or perpendicular to the thermal
and composition gradients that are parallel to the gravitational field. We
provide approximate solutions for all the relevant modes involved, which are
driven by gravity, conduction, and diffusion. We discuss the astrophysical
implications of our findings for a representative galaxy cluster where helium
has sedimented.
