Jan. 24, 2002
Vol. 21 No. 8

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    Experiments suggest silicon is an element in Earth’s core

    By Steve Koppes
    News Office

    Researchers in Geophysical Sciences used a diamond anvil cell to simulate high pressures found at Earth’s core.

    The chemical ingredients at the center of the Earth are surprisingly complicated, according to high-temperature, high-pressure experiments conducted by University scientists.

    The scientists, led by Jung-Fu Lin, a doctoral student in Geophysical Sciences, have found experimental evidence suggesting the Earth’s inner core consists largely of two exotic forms of iron instead of only one. These exotic forms of iron now appear to be alloyed with silicon, a lighter element. A previous study had once practically eliminated silicon as a candidate as the lighter element of the inner core.

    “Earth may not be quite as simple as we think in its very deepest parts,” said Dion Heinz, Associate Professor in Geophysical Sciences.

    Lin, Heinz and co-authors Andrew Campbell, Research Scientist in Geophysical Sciences; James Devine, doctoral student in Geophysical Sciences; and Guoyin Shen, Research Beamline Scientist at Argonne National Laboratory, reported their findings in the Friday, Jan. 11 issue of the journal Science. The study was conducted with support from the National Science Foundation and the National Aeronautics and Space Administration.

    Scientists had previously deduced that Earth’s core consists largely of iron.

    “Meteorites tell us that iron is a very abundant element in the solar system,” Heinz said. The Earth’s magnetic field further indicates that the core must be made of a conducting substance. Iron is the only element that is abundant enough and that also conducts at the high temperatures and pressures characteristic of Earth’s core.

    Seismologists have made further deductions about the characteristics of Earth’s core from the way that seismic waves from earthquakes and explosives travel through Earth.

    “They noticed that there has to be about 10 weight percent of a lighter element in the outer core and anywhere from 0 to 4 weight percent of a lighter element in the inner core,” Heinz explained.

    The lighter element in the core could be oxygen, sulfur, silicon, hydrogen or carbon.

    “Oxygen, sulfur and silicon are the three most-studied light elements,” Lin said. “Hydrogen and carbon aren’t well-studied, yet some studies have shown that these two elements can be ruled out because of their high-pressure properties.”

    A 1995 study by researchers at the University of California, Santa Cruz, seemed to rule out silicon, as well. That study was based on a compound containing an equal amount of iron and silicon.

    But it now appears that a lesser proportion of silicon is more appropriate for understanding the possible effect of silicon on the properties of iron under conditions at Earth’s core.

    The Chicago study now makes silicon the leading candidate, Lin said, because of its high level of abundance in the solar system, because it more readily alloys with iron and because of its ability to lower the density of iron under high pressure.

    The Chicago research team simulated searing subsurface temperatures of approximately 4,200 degrees Fahrenheit and crushing pressures of 840,000 atmospheres with a laser-heated diamond anvil cell. The late John Jamieson of the University and others at the National Bureau of Standards developed the diamond anvil cell in the late 1950s.

    A diamond anvil cell is designed to apply a large force to a very small sample squeezed between two diamonds. “One can reach to ultrahigh pressure with a small force,” Lin said.

    The pressures reached are so great, in fact, that sometimes the diamonds themselves shatter.

    “Diamond is not forever at all, yet a Science paper is priceless,” Lin said.

    The team identified the chemical composition of the samples they squeezed in the diamond anvil cell by using the Advanced Photon Source at Argonne National Laboratory. The Advanced Photon Source, the world’s most powerful source of X-rays, reveals the detailed microstructure of research materials.

    The team calibrated its findings by conducting an additional set of tests using the Advanced Photon Source and a large-volume press, a device that simulates pressures in the lower range of the diamond anvil cell.

    The atomic structure of iron changes under intense temperatures and pressures. The researchers found that iron might take on two different atomic structures in one tiny sample under conditions that would be found at a depth of more than 1,800 miles beneath Earth’s surface.

    The existence of two exotic forms of iron at Earth’s core could influence the interpretation of seismic data from there.

    “The only direct evidence about the core comes from seismic studies,” Heinz said. “Our experiments try to reproduce conditions in Earth’s deep interior. We compare the in situ measurements of the materials that we study with the seismic observations.”