Major Changes in Mineral Chemistry and Properties at High Pressures Seen

Embargoed for Release 6 P.M., EST January 30, 1997
NSF PR 97-5

Media contact:
Cheryl Dybas
(703) 306-1070

Program contact:
Robin Reichlin
(703) 306-1556

MAJOR CHANGES IN MINERAL CHEMISTRY AND PROPERTIES AT HIGH PRESSURES SEEN 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 planetís 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 weekís issue of the journal Science.

ìThis work is important,î says NSF earth sciences program director Robin Reichlin, ìbecause new crystalline structures at high pressure will lead to different sound velocities, and so affect scientistsí interpretation of seismic studies of the inner Earth. Metallic behavior also has important implications for modeling of Earthís magnetic field.î

ìThe question we addressed,î says Cohen, ìis 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 Earthís evolution.î

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 ìmagnetic collapseî 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.

-NSF-

Note to Reporters and Editors: Further information, color figures, and images are available at: http://granite.ciw.edu/-cohen/research/collapse.

NSF is an independent federal agency responsible for fundamental research in all fields of science and engineering, with an annual budget of about $3 billion. NSF funds reach all 50 states, through grants to more than 2,000 universities and institutions nationwide. NSF receives more than 50,000 requests for funding annually, including at least 30,000 new proposals. ** Receive NSF news releases and tipsheets electronically via NSFnews. To subscribe, send an e-mail message to [email protected]; in the body of the message, type "subscribe nsfnews" and then type your name. Also see NSF news products at: http://www.nsf.gov:80/od/lpa/start.htm and http://www.ari.net/newswise