The Core-Mantle Boundary

The core-mantle boundary, halfway down to the center of the Earth, has become a bit more understandable because of new laboratory studies of the behavior of rock under pressure and because of new computer simulations predicting the existence of another polymorph of the mineral MgSiO₃ that is more stable than the other phase previously known.

Previous seismic assessment of the so called D" layer just above the core-mantle boundary has been puzzling geoscientists. The most prevalent mineral at great depths is MgSiO₃, a mineral generally configured as a perovskite, a class of ceramic crystal in which three chemical elements in the ratio 1:1:3 form a distorted cubic structural unit. But some scientists believe that the perovskite cannot avoid dissociation amid the hard conditions at the core-mantle boundary.

One lab study of perovskites subjected to the conditions of high pressures and temperatures that approximate the D" layer, indicated that the mineral had survived in a new form. In other words, the great pressures and temperatures bring about a phase transition in the mineral. The scientists, at the Tokyo Institute of Technology, scattered x rays from their sample in its squeezed form.

The x-ray data has now been analyzed by collaborators at the University of Minnesota and the results, along with first principles calculations, were reported at last week's APS March Meeting in Montreal.

Minnesota scientists Jun Tsuchiya, Taku Tsuchiya, Koichiro Umemoto, and Renata Wentzcovitch (papers L28.9 and L28.11) said that the new form of MgSiO₃, called "post perovskite," should be stable at the D" layer. Its anisotropic structure, apparently unknown so far, could account for some of the seismic irregularities (changes in the speed of seismic waves) at those depths.