  We consider mass-imbalanced two-component Fermi gases for which the
unequal-mass atoms interact via a zero-range model potential with a diverging
s-wave scattering length $a_s$, i.e., with $1/a_s=0$. The high temperature
thermodynamics of the harmonically trapped and homogeneous systems are examined
using a virial expansion approach up to third order in the fugacity. We find
that the universal part of the third-order virial coefficient associated with
two light atoms and one heavy atom is negative, while that associated with two
heavy and one light atom changes sign from negative to positive as the mass
ratio $\kappa$ increases, and diverges when Efimov physics sets in at
$\kappa=13.61$. By examining the Helmholtz free energy, we find that the
equilibrium polarization of the trapped and homogeneous systems is 0 for
$\kappa=1$, but finite for $\kappa \ne 1$ (with a majority of heavy particles).
Compared to the equilibrium polarization of the non-interacting system, the
equilibrium polarization at unitarity is increased for the trapped system and
decreased for the homogeneous system. We find that unequal-mass Fermi gases are
stable for all polarizations.
