Equations of state of FeO and CaO

Abstract

New shock-wave (Hugoniot) and release-adiabatic data for Fe_(0.94)O 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 Fe_2O_3 or Fe_3O_4. The Hugoniot data for the B1 phases of FeO and CaO agree with the ultrasonically-determined bulk moduli (K_0= 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 K_0 and K′_0. Volume changes for B1/B2 transitions in oxides agree with theoretical expectations and with trends among the halides: -ΔV/V_1 ~ 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.