Oganov, Artem Romaevich; (2002) Computer simulation studies of minerals. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Applications of state-of-the-art computer simulations to important Earth- and rock-forming minerals (Al2SiO5 polymorphs, albite (NaAlSi3O8), and MgSiO3 perovskite) are described. Detailed introductions to equations of state and elasticity, phase transitions, computer simulations, and geophysical background are given. A new general classification of phase transitions is proposed, providing a natural framework for discussion of structural, thermodynamic, and kinetic aspects of phase transitions. The concept of critical bond distances is introduced. For Si-O bonds this critical distance is 2.25 Å. Using atomistic simulations, anomalous Al-Si antiordering in albite is explained. A first-order isosymmetric transition associated with a change in the ordering scheme is predicted at high pressures. A quantum-mechanical study is presented for the Al2SiO5 polymorphs: kyanite, andalusite, sillimanite, and hypothetical pseudobrookite-like and V3O5-like phases (the latter phase was believed to be the main Al mineral of the lower mantle). It is shown that above 11 GPa all the Al2SiO5 phases break down into the mixture of oxides: corundum (Al2O3) and stishovite (SiO2). Atomisation energies, crystal structures and equations of state of all the Al2SiO5 polymorphs, corundum, stishovite, quartz (SiO2) have been determined. Metastable pressure-induced transitions in sillimanite and andalusite are predicted at ~30-50 GPa and analysed in terms of structural changes and lattice dynamics. Sillimanite (Pbnm) transforms into incommensurate and isosymmetric (Pbnm) phases; andalusite undergoes pressure-induced amorphisation. Accurate quantum-mechanical thermal equation of state is obtained for MgSiO3 perovskite, the main Earth-forming mineral. Results imply that a pure-perovskite mantle is unlikely. I show that MgSiO3 perovskite is not a Debye-like solid, contrary to a common assumption. First ever ab initio molecular dynamics calculations of elastic constants at finite temperatures are performed for MgSiO3 perovskite. These for the first time allowed a physically sound interpretation of seismic tomography maps in terms of the temperature distribution in the lower mantle of the Earth.

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