Crystal Structures of Minerals at Ultrahigh Pressures and Temperatures

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A fundamental goal of modern geophysics is to understand the structure of planets and how they have evolved over geological time. This requires a detailed knowledge of the composition and mineralogy of different regions of the Earth's interior coupled with determination of some important chemical and physical properties such as compressibility, melting temperature, and rheology. However, the interiors of the Earth and planets are almost completely hidden from direct observation. As a result, it is necessary to infer properties of the deep Earth from geophysical observations, such as the variation of seismic wave velocity with depth inferred from earthquakes, and through laboratory experimentation. The latter is challenging as the Earth's deep interior is at pressures approximately one million times greater than that at Earth's surfaces and temperatures can reach up to 5000 K. The laser-heated diamond anvil cell is the only laboratory technique that can reproduce the pressure-temperature conditions in the deep mantle and core of the Earth. In this project, we will use this device in combination with synchrotron radiation facilities at the Advanced Photon Source to study the properties of a number of fundamental materials relevant to the deep mantle and core.For iron, molybdenum, and iron sulfide, we will carry out in situ x-ray diffraction experiments to conditions as high as 250 GPa and 4000 K. These results will provide new constraints on phases and equation of state of potential core materials, and will also aid in the interpretation of shock compression and static melting data. The recent discovery of a post-perovskite phase at conditions approaching the Earth's core-mantle boundary has major implications for understanding the structure and dynamics of this region. We will also carry out a comprehensive study of perovskite and post-perovskite phases in the analog system (Mg,Fe)GeO3-Al2O3 to 120 GPa and 2500 K. We will examine how compositional variables (Fe, Al) affect the equation of state, transition pressures, and structural response of both the perovskite and post-perovskite polymorphs in this system. Finally, we will examine SiO2 at ultrahigh pressures to test theoretical predictions of further phase transitions in silica. This research program will enhance research and education infrastructure by providing advanced training in high-pressure science, synchrotron x-ray techniques, mineral physics and global geophysics to graduate and undergraduate students. Our research will provide fundamental new knowledge about the physics of materials at high pressures and temperatures and the structure and state of planetary interiors