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Geoscientists know that Earth’s
core is mostly iron that slowly solidifies from the inside out, creating Earth’s
solid inner core. Researchers can estimate core density based on seismic observations
and the known density of pure iron. But even in the core’s high-pressure, high-temperature
environment, its density is 10 percent lower than the density of pure iron.
Therefore, some lighter element must also be present. Oxygen, carbon, sulfur,
hydrogen and silicon are all candidates.
In the Jan. 11 Science, a team of University of Chicago geophysicists
suggest that silicon alloys with iron in two high-pressure crystalline phases
in Earth’s inner core. Jung-Fu Lin and his colleagues studied the iron-rich
portion of iron-silicon alloys to better understand the possible crystal structures
and properties relevant to Earth’s core. Using the University of Chicago’s laser-heated
diamond anvil cell, they analyzed an iron alloy with 8 percent silicon by weight
at pressures of up to 84 gigapascals and temperatures up to 2400 Kelvin — not
quite the conditions in Earth’s core, which has an actual pressure of around
300 gigapascals and a temperature between 3500 and 6000 Kelvin.
Lin’s team found that alloying silicon with iron, even in small amounts, could
completely change the crystalline structure in the core. “Our work shows that
the lighter element in Earth’s core may not simply alloy with iron, but that
it can change the structure of the iron,” says Dion Heinz, one of the study’s
co-authors.
Current scientific consensus is that the inner core consists of hexagonal close-packed
iron crystals. Lin and others, however, open up the possibility that the inner
core could be a mixture of hexagonal close-packed (hcp) and body-centered cubic
(bcc) iron, where the silicon prefers the bcc phase. “What we thought was pretty
simple could actually be rather complex,” says Bruce Buffett, a geophysicist
at the University of British Columbia who studies Earth’s core and magnetic
field.
Although the study looked at an iron-silicon alloy with 8 percent silicon in
weight, the authors acknowledge that this percentage is most likely too high
for the actual core. “Somewhere between 4 and 8 weight percent silicon will
stabilize the bcc structure to high pressures and high temperatures, and this
phase coexists with the hcp structure,” Heinz says.
“The work is not really a smoking gun, saying everything has changed and the
inner core is going to be in fact bcc phase, or a mixture of the bcc and hcp,”
Buffett says. “What they’re pointing out is that small amounts of alloy components
can have these effects.” And Buffett says this could potentially affect our
current understanding of the generation of Earth’s magnetic field, travel of
P waves through the core, fractionation of elements in the core, and the core’s
thermal history and state. Future experiments, Heinz says, will need to test
the iron-phase properties with the other light elements that are candidate alloys.
Lisa M. Pinsker
This story first appeared as a Web
Extra on Jan. 11, 2002.
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