Minerals Behave Differently at High Pressures
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