D'' Layer Demystified

Strange stuff. Post-perovskite owes its odd crystal structure to the intense heat and pressure at the boundary between the mantle and core.

MONTREAL--Deep within Earth, where hellish temperatures and pressures create crystals and structures like none ever seen on the surface, a strange undulated layer separates the mantle and the core. The composition of this region, called the d" layer (pronounced "dee double prime"), has puzzled earth scientists ever since its discovery. Now, a team of researchers believes they know what the d" layer is.

Three thousand kilometers deep in Earth, the solid rock of the mantle meets the liquid outer core. At this juncture, seismic waves from earthquakes traveling through Earth suddenly change speed, and sometimes direction. These sudden shifts trace the border of the d" layer, which rises and falls in ridges and valleys. Researchers suspected that the layer marks a change in the crystal structure of the rock, which might happen at different depths depending on the temperature. This would explain the rises and dips of the boundary. But what could account for the sudden speed shifts of the seismic waves?

The explanation may lie in an entirely new kind of crystal structure, according to presentations by Jun Tsuchiya and Taku Tsuchiya here 23 March at a meeting of the American Physical Society. The researchers, from the University of Minnesota, Twin Cities, used a diamond anvil to squeeze and heat a grain of perovskite, the dominant mineral deep within Earth. They then took an x-ray image to see what happened to the molecular structure of the mineral in conditions like those in the d" layer. Only one crystal structure fit the x-ray data, and it was like nothing anyone had seen before.

The team dubbed the new structure "post-perovskite." It has a distinctive sandwich-like structure, and the team's calculations indicate that seismic waves would travel through it at different speeds depending on their initial direction--just like they do at the d" layer. And post-perovskite would form at different depths in Earth depending on the temperature, in agreement with the earlier predictions.

"This may explain the d" layer--it gives us a direction to look in," says Oliver Tschauner of the University of Nevada, Las Vegas. However, Surendra Saxena of Florida International University in Miami isn't convinced. He believes that perovskite falls apart near the d" layer, and that computer models of the type used by the Minnesota group can't properly predict that: "This theory isn't perfect yet."

Related sites
Renata Wentzcovitch's Web site
Explanation of inner Earth's structure