A gigahertz ultrasonic interferometer for the diamond anvil cell and high-pressure elasticity of some iron-oxide minerals

A second-generation high-frequency acoustic interferometer has been developed for high-pressure and high-temperature elasticity measurements in the diamond anvil cell. The instrument measures single-crystal compressional and shear-wave travel times, which are converted to sound velocities and elastic moduli for direct application to problems in geophysics. The second-order elastic constants (cij) of several iron-bearing oxide minerals has been measured under hydrostatic pressures to ~10 GPa. Pressure-induced c44 mode softening is observed in magnetite (Fe3O4), wustite (Fe0.95O) and in iron-rich magnesiowustite-(Mg, Fe)O, indicating that strong magnetoelastic coupling is common among these iron-rich oxides well ahead of known structural phase transitions. In (Mg, Fe)O, the pressure derivative of c44 is highly sensitive to composition and switches sign between 1.2 ± 0.2 at 25 mol% FeO to −0.96 ± 0.3 at 75 mol% FeO, and is about zero for (Mg, Fe)O containing 50 mol% FeO. In wustite, a discontinuity in the pressure derivatives c11 and C12 at ~5 GPa is interpreted to result from the onset of magnetic ordering, implying that a partially ordered but still cubic phase of FeO exists between ~5 GPa and where the rhombohedral distortion occurs at ~17 GPa.