A Different Spin

From October 13th, 2007; Vol.172 #15 (p. 237)

The Earth’s innards may steer seismic waves in stranger ways than scientists anticipated.

Jung-Fu Lin of Lawrence Livermore (Calif.) National Laboratory and his team performed a lab experiment on ferropericlase—a mixture of iron and magnesium oxides that makes up about 20 percent of Earth’s mantle. They found that in the conditions that exist from 1,000 to 2,200 kilometers below Earth’s surface, the material’s magnetic properties steadily change.

The gradual transition is due to a tighter packing of electrons in ferropericlase, which slightly increases its density, Lin says. For reasons that are poorly understood, the rearrangement also affects the propagation speed of seismic waves.

The researchers squeezed 12-micron-thick samples of ferropericlase between two diamond tips, subjecting the test material to pressures up to 1 million atmospheres. Meanwhile, they heated the samples with a laser to temperatures up to 2,000 kelvins and simultaneously probed them with a powerful X-ray beam. The X rays revealed a steady decrease in the iron oxide’s average spin, the quantum property that’s the equivalent of a bar magnet’s strength. The results appear in the Sept. 21 Science.

Lin says that iron atoms in ferropericlase have six outer electrons that normally spread over five different orbitals. At high pressures, however, the electrons pack tighter, with two occupying each of three orbitals. But two electrons can share an orbital only if they have opposite spins, so that in the high-pressure state, the iron atoms’ spins all cancel out.

To interpret seismic data, scientists focus on the mantle’s presumed composition, temperature, and pressure. “Now we’ll have to take spin into account,” says Lin.

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