Cesium chloride versus nickel arsenide as possible structures for MgO and FeO in the lower mantle.

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 to occur at pressures greater than 3000kbar, which implies that neither transition is likely for MgO in the lower mantle. For FeO, the transition to nickel arsenide is predicted to occur at much low pressure (approx 300kbar) than that to cesium chloride (3000kbar), and it is suggested that the transition observed by Jeanloz and Ahrens in shock experiments near 700kbar 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.-Authors