A new high-pressure phase

Of a perovskite may solve some mysteries near the core-mantle boundary halfway to Earth’s center. Just above that boundary, the so-called D″ layer exhibits a puzzling seismic discontinuity in which an elastic wave’s speed varies significantly with direction. When scientists from the Tokyo Institute of Technology placed the perovskite form of MgSiO₃ in a diamond anvil cell and subjected it to temperatures and pressures similar to those of the D″ layer, the mineral changed to a new crystalline form. By running molecular-dynamics simulations, the researchers found that only a slightly denser and highly anisotropic crystalline structure, dubbed post-perovskite, could fit their x-ray data. Because perovskite is plentiful in the deep mantle, the researchers say that the newfound form can explain some of the seismic irregularities. Using the Tokyo x-ray data, a University of Minnesota group led by Renata Wentzcovitch ran independent simulations and presented similar conclusions at the March meeting of the American Physical Society in Montreal. (M. Murakami et al., Science 304, 855, 2004 http://dx.doi.org/10.1126/science.1095932 ; T. Tsuchiya et al. , Earth Planet. Sci. Lett., in press.)