The phase transition of pure MgSiO_3 perovskite (Pbnm) to
the post-perovskite (Cmcm) structure has been recently
reported to occur at pressures and temperatures corresponding
to the Earth’s lowermost mantle [Murakami et al. 2004,
Tsuchiya et al. 2004, Oganov and Ono 2004]. We use ab initio
calculations to assess whether this transition survives, in the
Earth, for more realistic mantle compositions containing Al,
Fe^(2+), and Fe^(3+). We estimate phase coexistence pressures as
functions of minor element concentration, and from this we
obtain the effects of Al and Fe on the depth and sharpness of
the transition. For a pyrolitic mantle composition, with all the
Al partitioned into MgSiO_3, we find that Al preferentially
partitions into perovskite, and increases the transition pressure
by approximately 5 GPa. The transition takes place over a
depth range of width 225 km. Fe competes with Al by
lowering the transition pressure [Mao et al. 2004], so that
post-perovskite is likely to exist in the lower mantle.
However, the transition is still smooth, and not likely to
explain the sharp discontinuities observed seismically at the
base of the mantle. Our results suggest the geodynamical
implications of the post-perovskite phase transition require reevaluation