Structure and elasticity of a lower mantle oxide (magnesium,iron)oxygen and a new method of generating shear waves for gigahertz ultrasonic interferometry

Knowledge of the elastic properties and crystal chemistry of dense oxide structures plays an important role in interpreting the composition and mineralogy of Earth and planetary interiors from seismological observation. Magnesiowüstite-(Mg _x,Fe_1−x)O, with the rocksalt structure is expected to coexist with silicate perovskite-(Mg _y,Fe_1−y)SiO ₃ in Earth's lower mantle (660–2900 km depth) and is thus potentially the most abundant non-silicate oxide in Earth, although the exact composition (x and y) of the lower mantle is yet unknown. The effects of (Mg,Fe)2+ substitution and non-stoichiometry due to Fe3+ on the crystal structure, elastic constants ( c_ij), and adiabatic moduli (K_0S , G₀) of synthetic single-crystal (Mg,Fe)O are reported for the first time. In addition to periclase-MgO and wüstite-Fe _0.95O, nine different intermediate compositions were studied with varying ΣFe/(ΣFe+Mg) and ferric-iron contents (Fe3+/ΣFe) ranging from 1 to 12%. Compressional (P) and shear (S) elastic-wave velocities were measured at ambient pressure using GHz-ultrasonic interferometry with a new method of generating high-frequency shear waves. The new acoustic technique features a P-to-S conversion inside a single-crystal buffer rod, and produces the highest frequency shear waves (up to 2 GHz) ever made in the laboratory. Single-crystal structure refinements confirm that ferric iron (Fe3+) is partially accommodated into the rocksalt structure by occupancy of the normally vacant tetrahedral interstitial site. The c₁₁ and c₄₄ elastic constants soften from periclase to wüstite whereas the c₁₂ elastic constant increases. The rate of change in the elastic constants with composition is greatest between MgO and (Mg,Fe)O with ∼25 mol% FeO, such that adding Fe into periclase has a greater effect on the elastic properties than adding Mg to wüstite. The elastic anisotropy of (Mg,Fe)O has rather unusual behavior, being essentially constant for the range 0–25 mol% FeO, but then decreases linearly with Fe-content such that wUstite is elastically isotropic. The elastic properties of (Mg,Fe)O having similar total-Fe, but varying Fe3+ contents are identical within uncertainty. The isothermal compressibility of samples with 27, 56, and 75 mol% FeO was determined by X-ray diffraction in a diamond anvil cell at pressures up to ∼9 GPa resulting in determination of the bulk modulus: K_0T = 158.4(4), 155.8(9), and 151.3(6) GPa with ∂K_T/∂P = 5.5(1), 5.5(1), and 5.6(2), respectively. The [100] P-wave velocity was measured in the sample with ∼50 mol% FeO up to 3.6 GPa resulting in ∂ c₁₁/∂P = 7.6(4), being about 20% lower than that for MgO. Therefore, Fe has an overall softening effect on the elasticity of (Mg,Fe)O, at least up to 4 GPa. Finally, the first single-crystal ultrasonic measurements have been made at simultaneous high temperature and hydrostatic pressure in a diamond anvil cell. The [001] P-wave travel time was measured in San Carlos olivine (Mg_0.9Fe_0.1) ₂SiO₄ to temperatures of 250°C from a starting pressure of 2.5 GPa. Although the pressure could not be measured at temperature, this experiment illustrates that acoustic coupling between the diamond anvil and the sample can be maintained at high temperature in a liquid pressure medium, potentially motivating future ultrasonic measurem...