The propagation of seismic waves in the Earth's seismic wave velocities [Kanamori and Anderson, 1977]. The attenuation of seismic waves and the associated velocity dispersion in rocks are likely to be due to 1Now at Department of Geology and Geophysics, University of Minnesota, Minneapolis.relaxation process(es). In many cases these processes involve the motion of defects (including melts) in rocks, and therefore Earth's internal friction and velocity dispersion depend on the concentration, geometry, and mobility of relevant defects [e.g., Jackson and Anderson, 1970]. These defect-related properties are known to be sensitive to the thermochemical environment (temperature, pressure, fugacity of oxygen and water, etc.) and the differential stress, both of which are also closely related to tectonic activity. Therefore the observed seismic wave attenuation (and velocity anomalies), if properly interpreted, can provide important clues to Earth's thermochemical and stress states. The principal aim of

[1] Knowledge of the elastic properties and crystal chemistry of dense oxide structures plays an important role in interpreting the composition and mineralogy of planetary interiors. We report the effects of (Mg,Fe) 2+ substitution and nonstoichiometry due to Fe 3+ on the crystal structure, elastic constants (c ij ) and moduli (K 0T , K 0S , G 0 ) of (Mg,Fe)O utilizing single-crystal X-ray diffraction and gigahertz ultrasonic interferometry with a new method of generating high-frequency shear waves. The new acoustic technique features a P-to-S conversion by internal reflection on the oriented facet of a single-crystal MgO buffer rod. In addition to periclase (MgO) and wüstite (Fe 0.95 O), we investigated Fe 3+ -bearing (Mg,Fe)O single crystals prepared by interdiffusion having AEFe/(AEFe + Mg) = 0.06, 015, 0.24, 0.27, 0.37, 0.53, 0.56, 0.75, and 0.79, with ferric iron contents ranging from $1 to 12% of the total Fe. The elastic constants (c 11 , c 12 , c 44 ) are determined from compressional and shear wave velocities in the [100] and [111] propagation directions in the range of 0.5 -1.2 GHz. The c 11 and c 44 elastic constants soften from periclase to wüstite, whereas the c 12 elastic constant increases. The rate of change in the elastic constants with composition (@c ij /@x) is greatest between MgO and (Mg,Fe)O with $25 mol % FeO implying that substitution of 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 wüstite is elastically isotropic. The elastic properties of (Mg,Fe)O having similar total Fe but varying Fe 3+ contents are identical within uncertainty. The isothermal compressibility of samples with AEFe/(AEFe + Mg) = 0.27, 0.56, and 0.75 is determined by single-crystal X-ray diffraction in a diamond anvil cell to $9 GPa. For these samples, 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 (where values in parentheses indicate standard deviations). The deviation of @K T /@P from 4.0 corresponds to a difference in calculated density of about one percent for ferropericlase (Mg 0.8 Fe 0.2 )O at 30 GPa from the value predicted by second-order truncation of the BirchMurnaghan equation of state.