"Our results have extensive and interdisciplinary impact for the geophysics community." ~Taku Tsuchiya

Because we reported an important theoretical discovery related to MgSiO3 perovskite, the major Earth-forming mineral. This work provided theoretical support for the discovery by Murakami et al of this phase transition produced by in situ diamond anvil experiments ("Post-Perovskite Phase Transition in MgSiO3," Science7: May 2004: Vol. 304, No. 5672, pp. 855-858). Our paper, however, reported on the thermodynamic phase boundary, indicating that this transition occurs in the Earth’s mantle and could be the cause of the enigmatic D" seismic discontinuity in the lowermost mantle. This had not previously been clearly shown experimentally. Since the discovery of the parent phase in 1974, this has been the most important discovery in the field of mineral physics. Our results have extensive and interdisciplinary impact in the field of geophysics.

MgSiO3 perovskite is the major mineral component in the Earth’s lower mantle (from 660 km depth to the core-mantle boundary (CMB) at 2890 km depth). Our study confirmed that this mineral transforms into a new form, MgSiO3 post-perovskite, and showed that this occurs at pressures and temperatures expected at 200-300 km above the Earth’s CMB. Below these depths, there is a layer of different chemical/mineralogical make-up, the D" (dee-double-prime) layer. Seismic velocities change discontinuously at the boundary of this layer defining the so-called D" discontinuity. Its origin was very unclear. The newly found post-perovskite transition appears to explain, at least in part, these velocity changes. MgSiO3 post-perovskite offers a new paradigm for interpreting properties of the D" layer.

  How did you become involved in this research?

We received a copy of the unidentified X-ray diffraction pattern obtained by Motohiko Murakami and Kei Hirose from the Tokyo Institute of Technology, before the new phase was identified and became public. We all became highly motivated and started very early searching theoretically for a crystal structure that could explain the experimental finding. We were in a particularly special position to conduct this research because the computational techniques that had been developed by Renata during the past 15 years permitted reliable and predictive investigations of materials under extreme pressures and temperatures.

  What are the social or political implications of your research?

The scientific implications are huge and interdisciplinary. The discovery of this phase transition is one of the most important advances in geophysics in decades. It sets new directions for research and scientific funding. This discovery also arouses the interest of other scientific fields, such as that of materials science, crystallography, etc., since perovskite-related materials are quite common in many areas of applied science. In fact this structural change has already been found in several other perovskite materials since then. Culturally/socially, it points to the major role that the field of materials physics is playing within the field of geophysics. This discovery was made outside a geophysics department. It is pure theoretical materials science. This trend that had been developing throughout the last decade is now undeniable. Indeed, it is even changing the way in which theoretical mineral physicists are being trained.

Taku Tsuchiya, Ph.D.
Associate Professor of Theoretical and Computational Mineral Physics 
Geodynamics Research Center 
Ehime University
Matsuyama, Japan

Renata Wentzcovitch, Ph.D.
Professor 
Chemical Engineering and Materials Science 
Institute of Technology 
University of Minnesota 
Minneapolis, MN, USA