Applications of Density Functional Theory in the Geosciences
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Applications of Density Functional Theory in the Geosciences John P. Brodholt and L. Voc`´adlo Abstract Although density functional theory (DFT) calculations have been widely used in many areas of the geosciences for the last 15 years, arguably the most successful application of these methods has been when they are used to understand the properties of minerals and melts in the Earth’s interior. This is simply because the temperatures and pressures of the Earth’s mantle and core (up to ⬃6000 K and 360 GPa) are so extreme that experiments under these conditions are very difficult. DFT calculations have been used to provide invaluable estimates of physical parameters that are fundamental to understanding the dynamics and evolution of the Earth. In particular, DFT calculations have helped provide estimates of the mineralogy and chemistry of the Earth’s core, the high-temperature and pressure elasticity of the stable crystal phases in the Earth’s mantle, the effect of defects on physical properties of mantle minerals, and, most recently, the discovery of a new phase of (Mg,Fe)SiO3 just above the core. These and other applications of DFT in the geosciences are described and their implications discussed. Keywords: ab initio, core, Earth, geologic, high-pressure, mantle.
Introduction The surface of the Earth exhibits many features and processes that directly reflect processes originating in the Earth’s interior. Volcanic eruptions and earthquakes, for instance, are the violent and impressive manifestations of large-scale convective processes that occur because the Earth cannot release its heat by conduction alone. The magnetic field that, among other things, protects us from harmful solar rays and provides a navigational tool is also the result of large-scale convection, in this case in the Earth’s liquid outer core. The time scale of these processes is far longer than the sudden events (earthquakes, volcanic eruptions) that have such an impact on humans, but nevertheless, to understand them properly we must understand the underlying properties and processes causing them. There is no realistic chance of ever directly sampling much
MRS BULLETIN • VOLUME 31 • SEPTEMBER 2006
of the interior of the Earth and certainly no chance of visiting it. All of our understanding of the inner regions of the planet is, therefore, the result of interpreting remote measurements. Of these, seismology has certainly had the greatest impact and continues to provide increasingly detailed and precise data on the seismic structure of the mantle and core. But taken alone, seismic data are of little use without the mineral physics data to interpret them. To do this, the elastic properties of all the possible minerals across the full range of pressures and temperatures in the Earth must be known. The interior could then be mapped out in terms of mineralogy, composition, and temperature (Figure 1). But to go further, transport properties such as diffusion, viscosity, and thermal-conductivity data are needed to make inferences about the dynamical beh
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