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NSF PR 97-5 - January 30, 1997
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Major Changes in Mineral Chemistry and Properties
at High Pressure 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 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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