The effect of minor elements on the physical and chemical properties of lower mantle minerals at high-pressure

The sound velocities, elasticity, and crystal chemistry of minerals in Earth's lower mantle are of general importance for the interpretation of seismic wave observations, geochemical modeling, and geodynamic simulations. Aluminum-bearing (Mg,Fe)SiO3 perovskite and (Mg,Fe)O ferropericlase are proposed to be the most abundant minerals in Earth's lower mantle, with aluminum-bearing (Mg,Fe)SiO3 perovskite occupying roughly half of Earth's volume. Despite their major roles in the deep Earth, our knowledge about the sound velocities and crystal chemistry of these chemically complex materials under the appropriate pressure-temperature conditions is still quite limited. I have contributed to the understanding of these problems by applying Brillouin spectroscopy to determine the sound velocities, as well as novel nuclear resonant scattering of synchrotron radiation to investigate the electronic state of iron (which is thought to influence material properties) in candidate lower mantle minerals. I performed high-pressure Brillouin spectroscopy measurements to determine the sound velocities and elasticity of polycrystalline aluminous MgSiO3 perovskite (containing 5 wt.% Al2O3) to 45 GPa and single-crystal (Mg0.94Fe0.06)O ferropericlase to 20 GPa. The measurements were made with diamond anvil cells using methanol-ethanol-water or neon as pressure-transmitting media. The results, in combination with a one-dimensional average Earth model (PREM), show that the average mineralogy of Earth's lower mantle should be enriched in Si compared to a peridotitic upper mantle. I also conclude that observed seismic lateral variations in Earth's lower mantle could be caused by a variation in the aluminum content on silicate perovskite, based on knowledge of the shear properties. In addition to sound velocity measurements, I carried out synchrotron Mossbauer spectroscopy measurements of (Mg,57Fe)SiO3 perovskite to 120 GPa. The electronic properties, including the charge and spin states, of iron in silicate perovskite were determined to 120 GPa.