MgSiO3-rich perovskite-type phase is the most abundant mineral inside the Earth. This paper reports a novel phase transition from MgSiO3 perovskite to a new high-pressure form that possesses CaIrO3-type structure above 125 GPa and 2500 K. It indicates that this new phase, called as "post-perovskite phase," is a major constituent mineral at ~2600-2900-km depth, the bottom several hundred kilometers of the Earth’s mantle.

“This work is probably the most important discovery in high-pressure mineral physics since 1974 when the MgSiO3 perovskite phase was first synthesized at a pressure of 30 GPa.”

This paper has attracted the interest of many people because a variety of seismic anomalies may be reconciled with this new mineral. The Earth’s lower mantle (660-2900-km depth) is a seismically "quiet" region except at its bottom. In contrast, the large velocity discontinuity and shear-wave anisotropy have been observed in the lowermost mantle, called the D" region. These anomalies could not be explained with the known mantle minerals.

This new phase transition may have large effects on the style of mantle convection, depending on the pressure/temperature slope of the phase transition boundary. The topography of the seismic discontinuity at the D" region suggests a positive pressure/temperature slope. It is also supported by the most recent theoretical calculations. If it is true, the post-perovskite phase transition activates the heat transport from the base of the mantle that directly contacts with the hotter metallic core. Results of computer simulation studies on mantle convection suggest that temperature of the uppermost mantle increases by several hundred degrees when taking this phase transition into account.

This work is probably the most important discovery in high-pressure mineral physics since 1974 when the MgSiO3 perovskite phase was first synthesized at a pressure of 30 GPa. Phase transition to the post-perovskite phase is a kind of surprise, because a pressure-induced phase transition from perovskite to denser structure was not known in ABX3 composition. This phase transition has never been proposed by theory.

Our experimental technique to synthesize material at high pressure and temperature is useful not only for Earth science but also for new materials research. Like graphite to diamond transformation, all materials undergo phase transition to high-pressure forms with increasing pressure. The high-pressure phase sometimes shows completely different physical properties. Material synthesis is now possible at 250 GPa and 2000 K by the laser-heated diamond anvil cell (LHDAC) techniques.

The Earth’s mantle has a layered structure due to a structural change of its main constituent minerals. It has long been believed that the mantle is divided into four layers with three major phase transitions at 410, 520, and 660-km depths. Our discovery indicates that there is one more phase change at 2600-2700-km depth. The last phase transition and the fifth layer of the mantle were finally revealed by this study.

In addition, a long-term enigma in the deepest mantle may be reconciled with this new mineral. Since a number of large seismic anomalies cannot be explained with known mantle minerals, the lowermost mantle has long been the most enigmatic region in the Earth’s interior. It has often been discussed that these seismic anomalies are of thermal or chemical origin. Most of them can simply have been caused by the newly discovered post-perovskite phase.

MgSiO3 is a representative chemical composition of the Earth’s mantle. MgSiO3 adopts perovskite structure above 23 GPa, and is believed to be a predominant mineral, at least in the upper part of the lower mantle. However, the stability of MgSiO3 perovskite has never been confirmed in the deep lower mantle.

The pressure and temperature condition at the bottom of the mantle is 135 GPa and ~2500 K. It has been very difficult to generate such extreme conditions and collect x-ray diffraction data. We started x-ray experiments in 2001 at BL10XU of SPring-8, the largest synchrotron facility in the world. Due to a number of technical improvements, experiments over 100 GPa and 2000 K became possible recently. Only a few groups, including ours, are currently working at such conditions.

Our discovery largely owes to the above-noted technical improvements. We first performed x-ray diffraction experiments on MgSiO3 perovskite up to the condition at the base of the mantle. The phase transition of MgSiO3 perovskite was observed above 125 GPa and 2500 K.