Major Changes In Mineral Chemistry And Properties At High Pressure Seen

January 30, 1997

Implications for Structure and Chemical Evolution of the Earth

Changes in the magnetic structure of minerals at high pressures might have significant implications for the structure and evolution of the Earth, and may have a significant impact on the planet's magnetic field.

National Science Foundation (NSF)-funded scientists Ronald Cohen, Igor Mazin, and Donald Isaak performed computations at the Geophysical Laboratory of the Carnegie Institution of Washington (D.C.) which suggest that models for low-pressure chemical behavior may not be accurate at high pressures. The results are published in this week's issue of the journal Science.

"This work is important," says NSF 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 of Earth's magnetic field."

"The question we addressed," says Cohen, "is whether there are grounds for expecting mineral chemistry to undergo drastic changes at high pressures. Our computations predict such transitions in minor elements, such as cobalt, in the deep Earth that will affect geochemical models of Earth's evolution."

The researchers used a Cray J90 supercomputer at the Geophysical Laboratory purchased with major support from NSF.

Cohen, Mazin, and Isaak predicted collapse at high pressures of the magnetic state that characterizes certain materials at low pressures. Such "magnetic collapse" would lead to radical changes in the properties of these materials; for instance, they may become metallic or new crystal types or compositions may form.

The scientists also investigated the high-pressure properties of materials containing metal ions of iron, manganese, cobalt, and nickel, in order to understand the behavior of materials in the deep Earth. Direct measurements of the magnetic structure of minerals are very difficult at high pressures, say the scientists, because of the very small sample sizes and the fact that high-pressure instruments tend to contain metallic components.

The properties of such materials at low pressures are well understood; much of our understanding of minerals and rocks is based on low-pressure behavior. However, the scientists predict that at high pressures the magnetic structure of rocks and minerals breaks down, and they behave very differently. For example, iron and magnesium ions substitute for each other readily in low-pressure minerals. At high pressure, however, iron ions are very different from magnesium ions, and instead of mixing with magnesium, may form new iron-rich minerals.

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National Science Foundation

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