Discovery of post-perovskite phase transition in MgSiO₃ and the earth's lowermost mantle

MgSiO₃ perovskite is believed to be a dominant mineral at least in the upper part of the Earth's lower mantle, but its stability and possible phase transition in deeper levels were not known. Recently we discovered the phase transition from MgSiO₃ perovskite to a new high-pressure form (space group: Cmcm) above 125 GPa and 2500 K on the basis of in-situ x-ray diffraction measurements [1]. This phase transition is most likely responsible for the origin of the D" seismic discontinuity observed around 2700 km depth, and the MgSiO₃ post-perovskite phase is a main constituent mineral in the D" region. Here we introduce the details of high-pressure experiment and crystal structure determination, and discuss the seismic anomalies in the lowermost mantle.