The atomistic mechanism of the perovskite -> post-perovskite transformation of MgSiO3 is explored from molecular simulation mimicking extreme pressure and temperature conditions akin to planet Earth's interior (D '' region). The nucleation process is highly anisotropic and initiated by column-wise rotation of SiO6 octahedra around the [001] direction. The post-perovskite transition is found to be a rare event (i.e. one requiring substantial activation energy, similar to 15 eV for the critical nucleus, but, once started, propagates very fast (10(3)-10(4) m/s). The most stable phase front, the (010) interface, propagates at only 10% of the speed of the (001) phase front. (C) 2013 Elsevier B.V. All rights reserved.