High-pressure phases in the Al2SiO5 system and the problem of aluminous phase in the Earth's lower mantle: ab initio calculations

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 f orms its own minerals or is mainly contained in (Mg,Fe)SiO3-perovskite. Thi s question is very important, since it is known that if Al were mainly host ed by perovskite, it would radically change Fe/Mg-partitioning and phase eq uilibria 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 us to recon sider many of our geochemical and geophysical models for the lower mantle. This work considers the possibility of a V3O5-type structured modification of Al2SiO5 to be the main host of Al in the lower mantle, as proposed by pr evious workers. We report ab initio calculations, based on density function al theory within the generalised gradient approximation (GGA) with plane wa ve basis set and nonlocal pseudopotentials. We consider polymorphs of Al2Si O5 (kyanite, andalusite, sillimanite, and hypothetical V3O5-like and pseudo brookite-like phases), SiO2 (stishovite, quartz) and Al2O3 (corundum). Comp utational conditions (e.g., plane-wave energy cutoff, Brillouin zone sampli ng) were carefully chosen in order to reproduce small energy changes associ ated with phase transitions between the Al2SiO5 polymorphs. Good agreement of crystal structures, bulk moduli, atomisation energies and the phase diag ram of Al2SiO5 with experimental data was found. Strong disagreement betwee n the calculated lattice parameters and density of V3O5-like phase of Al2Si O5 and experimental values, assigned to it by previous workers, suggests th at a V3O5-structured phase of Al2SiO5 was never observed experimentally. In addition, we found that the most stable high-pressure assembly in Al2SiO5 system is corundum + stishovite, and the value of the transition pressure a t T = O K (113 kbar) is in excellent agreement with experimental estimates (95-150 kbar). We explain the instability of octahedrally coordinated silic ates of Al to decomposition on the basis of Pauling's second rule.