High-pressure phases in the Al_2SiO_5 system and the problem of aluminous phase in the Earth's lower mantle: ab initio calculations

摘要

One of the main uncertainties in mineralogical models of the Earth's lower mantle is the nature of the aluminous mineral: it is not clear whether Al forms its own minerals or is mainly contained in (Mg,Fe)SiO_3-perovskite. This question is very important, since it is known that if Al were mainly hosted by perovskite, it would radically change Fe/Mg-partitioning and phase equilibria between mantle minerals, and also alter many physical and chemical properties of perovskite, which is currently believed to comprise ca. 70% of the volume of the lower mantle. This, in turn, would require as to reconsider many of our geochemical and geophysical models for the lower mantle. This work considers the possibility of a V_3O_5-type structured modification of Al_2SiO_5 to be the main host of Al in the lower mantle, as proposed by previous workers. We report ab initio calculations, based on density functional theory within the generalised gradient approximation (GGA) with plane wave basis set and nonlocal pseudopotentials. We consider polymorphs of Al_2SiO_5 (kyanite, andalusite, sillimanite, and hypothetical V_3O_5-like and pseudobrookite-like phases), SiO_2 (stishovite, quartz) and Al_2O_3 (corundum). Computational conditions (e.g., plane-wave energy cutoff, Brillouin zone sampling) were carefully chosen in order to reproduce small energy changes associated with phase transitions between the Al_2SiO_5 polymorphs. Good agreement of crystal structures, bulk moduli, atomisation energies and the phase diagram of Al_2SiO_5 with experimental data was found. Strong disagreement between the calculated lattice parameters and density of V_3O_5-like phase of Al_2SiO_5 and experimental values, assigned to it by previous workers, suggests that a V_3O_5-structured phase of Al_2SiO_5 was never observed experimentally. In addition, we found that the most stable high-pressure assembly in Al_2SiO_5 system is corundum + stishovite, and the value of the transition pressure at T = O K (113 kbar) is in excellent agreement with experimental estimates (95-150 kbar). We explain the instability of octahedrally coordinated silicates of Al to decomposition on the basis of Pauling's second rule.