The Earth’s mantle is usually divided into three parts: the upper mantle, which extends to a depth of about 410 km below the surface; the transition-zone between 410 and 670 km; and the lower mantle, which extends to a depth of 2898 km. Although the mineralogy of the upper mantle and transition-zone are relatively well known, the lower mantle remains a mystery because no samples are available from this region.

To study the lower mantle, geophysicists must probe it remotely using seismic tomography. This technique produces 3D maps of seismic velocities and densities from which the properties of the mantle can be determined. However, such measurements have revealed features in the lowermost mantle that cannot be explained. The best known of these is a seismic discontinuity called the D" discontinuity that is found some 2700 km below the Earth's surface.

Motohiko Murakami of the Tokyo Institute of Technology and colleagues used X-ray diffraction to analyse artificially synthesised magnesium silicate at pressures up to 134 gigapascals and temperatures up to 2600 kelvin. These conditions correspond to those that are found at the D" discontinuity. They found that the magnesium silicate undergoes a phase transition in which it changes from a distorted cubic structure to a structure that contains stacked octahedral sheets of silicate.

According to Murakami and co-workers, this new “post-perovskite” structure -- which is anisotropic and stable at the pressures and temperatures studied -- can explain the existence of the D" discontinuity and other features of the lower mantle.