Complicating iron in the core
Geoscientists know that Earth's core is mostly iron that slowly freezes from inside out, creating the 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 at pressures 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 K.
They 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 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, how P waves travel through the core, how elements get fractionated 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