Newly Discovered Mineral In The Earth’s Mantle

The lowermost region of the earth’s mantle, the D” layer, has presented great problems to geophysicists for a long time. This irregularly bounded layer, which is about 150 kilometres thick on average, shows very different physical properties to those lying above. The reason why the Earth’s mantle at the interface to the core should have such anomalous structure and properties could not be convincingly explained until now. Artem R. Oganov from the Laboratory for Cristallography at ETH Zurich and Shigeaki Ono from the Japan Agency for Marine-Earth Science and Technology now report in the latest issue of the journal “Nature” that the D” layer must consist of a previously unknown mineral.

New interpretation of observations

The discovery of the mineral throws a new light on the processes in the Earth’s mantle and is likely to have a significant influence on geophysical research. The D’’ layer, for example, plays an important role in the genesis of hot mantle plumes. These mantle plumes are for instance responsible for the volcanism on Iceland or Hawaii. Because of the new discovery, many observations pertinent to the Earth’s mantle must now be re-interpreted.

Surprising crystal structure

On the basis of quantum-mechanical calculations and high-pressure experiments, the two researchers were able to show that the mineral perovskite, the most important component of the lower mantle of the Earth, transforms under extreme pressure to a post-perovskite phase. This previously unknown mineral exhibits the same chemical composition as perovskite but has a peculiar layered crystal structure. The appearance of such a structure at high pressure is very unusual and came as a surprise to the researchers. With this new crystal structure, one can now explain why the rocks of the D” layer are so anisotropic and show such different seismic properties from the mantle rocks lying above, which consist of the almost isotropic mineral perovskite.

Temperature dependence

The D” layer possesses an unusual topography, which the two researchers can now explain. The pressure at which perovskite is converted into post-perovskite strongly depends upon temperature. Depending on how hot the mantle rocks are, the phase transition takes place at a different depth. Wherever the Earth’s mantle is specially hot, the D” layer is only a few dozen kilometres thick. In the “cooler” regions, on the other hand, it can extend to 300 kilometres.