Pressure-induced changes in the compression mechanism of aluminous perovskite in the Earth's mantle

Although aluminium is the fifth most abundant element in the Earth's mantle , its effect on the physical properties of perovskite, the main mineral pha se in the lower mantle, has largely been ignored. It is becoming clear, how ever, that many properties of MgSiO3 perovskites are remarkably sensitive t o small amounts of aluminium(1-4). In particular, perovskite with only 5 wt % Al2O3 has a bulk modulus 10% lower than that of the pure magnesian end-me mber(12). The increased compressibility may be due to the high concentratio ns of oxygen vacancies required to balance the charge of the aluminium 5; i f so, this would have important consequences for the mantle, as aluminous p erovskites could be weaker, have lower seismic velocities and be hosts for water. To test whether oxygen vacancies exist in aluminous perovskites, I h ave calculated the compressibility of end-member defect-bearing perovskites using ab initio methods. The results show that perovskites with oxygen vac ancies do have significantly greater compressibilities than those without s uch vacancies. But the results also suggest that oxygen vacancies become un favourable at high pressures, in which case only the physical properties of the shallow lower mantle would be affected by aluminium-with the deeper ma ntle retaining properties similar to those of aluminium-free perovskite.