Thermochemical systematics are used to estimate the pressure for the transition from rocksalt to cesium chloride and from rocksalt to nickel arsenide structures. For MgO, both transitions are predicted is occur at pressures greater than 3000 k bar, which implies that neither transition is likely for MgO in the lower mantle. For FeO, the transition to nickel arsenide to predicted to occur at much lower pressure (~300 kbar) than that to cesium chloride (3000 kbar), and it is suggested that the transition observed by Jeanloz and Ahrens in shock experiments near 700 kbar may be to the nickel arsenide structure. If a (Mg, Fe)O solid solution with NiAs structure exists in the lower mantle as a minor phase, it will be strongly enriched in iron and other transition metals relative to a (Mg, Fe) SiO3 perovskite phase. This enrichment in the heavier elements, as well as the slightly higher density of nickel arsenide than rocksalt phases, would accentuate the density difference between MSiO3 and MO phases and enhance gravitational settling and lower mantle heterogeneity.