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Minerals Behave Differently at High Pressures

January 1998

Geologists are quite certain about iron.

They know, for example, that it is a versatile magnetic metal with many uses, and that in most of the minerals on the Earth's surface iron oxidizes into ions. Iron ions that are missing two electrons are called ferrous iron, and those missing three electrons are called ferric iron. Even the fact that ferrous iron easily substitutes with magnesium, doesn't puzzle anyone too much; ferrous iron and magnesium ions are very similar in size.

However, geologists' certainty about iron is only as deep as the Earth's surface. Closer to Earth's center, the pressure becomes intense, causing iron, along with other minerals, to lose its familiar characteristics.

In short, iron's magnetic properties collapse. This is one of the predictions made by NSF-funded geophysicists Ronald Cohen from the Carnegie Institution of Washington, Igor Mazin now at George Mason University, and Donald Isaak now at University of California in Los Angeles. The results of their computational models appeared in the journal Science.

Overall, the team's research has implications for future work in high pressure physics and chemistry. The study suggests that low-pressure chemical concepts may not be useful at high pressures, and that mineral chemistry could be significantly different at high pressures.

"This work is important,"says NSF's Earth Sciences Program Director Robin Reichlin, "because new crystalline structures at high pressure will lead to different sound velocities, and so affect scientists' interpretation of seismic studies of the inner earth. Metallic behavior also has important implications for modeling Earth's magnetic field."

Currently, researchers studying the inner earth, the area about 1,864 miles below the surface, use seismic waves to map that expanse. They interpret the movement of the waves to gauge both the kinds of materials that are in the inner earth, and where those materials are located.

However, if the minerals do not behave as they do on the Earth's surface, the waves travel through them at different speeds than at the surface, and the interpretation of the waves will have to change.

The team also investigated the origin of magnetic collapse, which had been discussed for decades but was still poorly understood. Cohen says he and his colleagues were able to investigate this phenomenon by using the Carnegie Institution's supercomputer, which is supported by NSF, and by focusing on crystalline structures instead of individual atoms.

The team's theoretical work will be used by experimental scientists studying high pressure systems, says Cohen. While experimental verification is rare, in recent years, another of Cohen's predictions was verified in high pressure experiments performed at the NSF-supported Center for High Pressure Research.

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