Dolomite is a major constituent of subducted carbonates; therefore evaluation of its phase stability and equation of state at high pressures and temperatures is important for understanding the deep Earth carbon cycle. X-ray diffraction experiments in the diamond anvil cell show that Ca(0.988)Mg(0.918)Fe(0.078)Mn(0.016)(CO(3))(2) dolomite transforms to dolomite-II at similar to 17 GPa and 300 K and then upon laser-heating transforms to a new monoclinic phase (dolomite-III), that is observed between 36 and 83 GPa. Both high-pressure polymorphs are stable up to 1500 K, indicating that addition of minor Fe stabilizes dolomite to Earth's deep-mantle conditions. Citation: Mao, Z., M. Armentrout, E. Rainey, C. E. Manning, P. Dera, V. B. Prakapenka, and A. Kavner (2011), Dolomite III: A new candidate lower mantle carbonate, Geophys. Res. Lett., 38, L22303, doi:10.1029/2011GL049519.