Ice freezing of aqueous ammonium sulfate particles containing hematite or corundum mineral dust cores is studied by aerosol flow tube infrared spectroscopy (AFT-IR). The cores induce freezing heterogeneously at temperatures warmer than homogeneous nucleation. Heterogeneous nucleation rates (j) vary from 10(2) to 10(6) cm(-2) s(-1), depending on the mode diameter of the hematite or corundum core (50-250 nm), temperature, and aqueous mole fraction composition. The rates are rationalized by the equations of classical heterogeneous nucleation theory, which yields contact angles of 90degrees for ice/hematite and ice/corundum interfaces and temperature-dependent (215-235 K) surface tensions of ice against aqueous ammonium sulfate solutions. The slope of the temperature dependence is positive for pure water but is progressively negative as the ammonium sulfate content increases. These results quantify a potentially important role for mineral dusts as initiators of cirrus cloud formation by heterogeneous nucleation.