Equations of state of FeO and CaO

Abstract

New shock-wave (Hugoniot) and release-adiabatic data for Fe0.94O and CaO, to 230 and 175 GPa (2.3 and 1.75 Mbar) respectively, show that both oxides transform from their initial B1 (NaCl-type) structures at about 70 (±10) GPa. CaO transforms to the B2 (CsCl-type) structure and FeO is inferred to do the same. Alternatively, FeO may undergo an electronic transition, but it probably does not disproportionate under shock to Fe and Fe2O3 or Fe3O4. The Hugoniot data for the B1 phases of FeO and CaO agree with the ultrasonically-determined bulk moduli (K0= 185, 112 GPa, respectively) and with the ultrasonically-determined pressure derivative for CaO (K′0= 4.8); K′0∼ 3.2 for FeO is determined from the present data. The Hugoniot data for both FeO and CaO are consistent with low- and high-pressure phases having identical K0 and K′0. Volume changes for B1/B2 transitions in oxides agree with theoretical expectations and with trends among the halides: -ΔV/V1∼ 4 per cent and 11 per cent for FeO and CaO respectively. Also, the transition pressures increase with decreasing cation/anion radius ratio for the oxides. The Hugoniot data show that the density of the outer core is equal to that of a 50–50 mix (by weight) of Fe and FeO (∼10 wt per cent oxygen), consistent with geochemical arguments for the presence of oxygen in the core. In terms of a mixture of simple oxides, the density of the lower mantle is satisfied by Fe/(Mg + Fe) ∼ 0.12, however, arbitrarily large amounts of CaO can be present; an enrichment of refractory components in the lower mantle is allowed by the shock-wave data. Because of the relatively low transition pressure in FeO, a B1/B2 transition in (Mg, Fe)O is likely to occur in the lower mantle even if MgO transforms at 150–170 GPa. Such a transition may contribute to the scattering of seismic waves and change in velocity gradient found near the base of the mantle.