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
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.