Diamonds in Earth Science

To clarify the situation, experiments would need to push above 120 Gigapascal and 2500 Kelvin. I [...] started laboratory experiments using diamond-anvil cell, in which samples of mantle-like materials are squeezed to high pressure between a couple of gem-quality natural diamonds (about two tenths of a carat in size) and then heated with a laser. Above 80 Gigapascal, even diamond—the hardest known material—starts to deform dramatically. To push pressure even higher, one needs to optimize the shape of the diamond anvils's tip so that the diamond will not break. My colleagues and I suffered numerous diamond failures, which cost not only research funds but sometimes our enthusiasm as well.
(From The Earth's Missing Ingredient)

But in the end, Kei Hirose and his group succeeded in subjecting a small sample of magnesium silicate to the pressure and temperature that prevails in the lower Earth's mantle, about 2700 kilometer below our feet.

Planet Earth has an onion-like structure, as has been revealed by the analysis of seismological data: There is a central core consisting mostly of iron, solid in the inner part, molten and liquid in the upper part. On top of this follows the mantle, which is made up of silicates, compounds of silicon oxides with magnesium and other metals. The solid crust on which we live is just a thin outer skin.

The lower part of the mantle down to the iron core was long thought to consist of MgSiO₃ in a crystal structure called perovskite. However, seismological data also revealed that the part of the mantle just above the CMB (in earth science, that's the core-mantle boundary, not the cosmic microwave background... ) somehow is different from the rest of the mantle. This lower-mantle layer was dubbed D″ (D-double-prime, shown in the light shade in the figure), and it was unclear if the difference was by chemical composition or by crystal structure.

As Kei Hirose describes in the June 2010 issue of the Scientific American, his group started a series of experiments to study the properties of magnesium silicate at a pressure up to 130 Gigapascal (water pressure at an ocean depth of 1 kilometer is 0.01 GPa) and a temperature exceeding 2500 Kelvin ‒ the conditions expected for the D″ layer of the lower mantle.

To achieve such extreme conditions, one squeezes a tiny piece of magnesium silicate between the tips of two diamonds, and heats up the probe by a laser. The press used in such experiments is called "laser-heated diamond anvil cell".

(From "Laser-heated diamond anvil cell technique: double sided heating with multimode Nd:YAG laser" by G. Shen, H.K. Mao, and R.J. Hemley.)

The figure shows the core of a diamond anvil cell: The sample to be probed is fixed by a gasket between the tips of two diamonds. The diameter of the tips is about 0.1 millimeter, so applying a moderate force results in huge pressure.

Diamonds are used because of their hardness, but they have the additional bonus of being transparent. Hence, the probe can be observed, or irradiated by a laser for heating, or x-rayed for structure determination.

The diamonds are fixed in cylindrical steel mounts, but creating huge pressure does not require huge equipment: The whole device fits on a hand! (Photo from a SPring-8 press release about Kei Hirose's research.)


Actually, the force on the diamond tips is applied in such a device by tightening screws by hand.

In the experiment, the cell was mounted in a brilliant, thin beam of x-rays created by the SPring-8 synchrotron facility in Japan. This allows to monitor the crystal structure of the probe by observing the pattern of diffraction rings.

It was found that under the conditions of the D″ layer of the lower mantle, magnesium silicate forms a crystal structure unknown before for silicates, which was called "Post-Perovskite". The formation of post-perovskite in the lower mantle is a structural phase transition of the magnesium silicate, and this transition can explain the existence of a separate the D″ layer, and many of its peculiar features. It also facilitates heat exchange between core and mantle, which seems to have quite important implications for earth science.

And here is the heart of the experiment (from the "High pressure and high temperature experiments" site of the Maruyama & Hirose Laboratory at the Department of Earth and Planetary Sciences, Tokyo Institute of Technology) ‒ a diamond used in a diamond anvil pressure cell:


High-quality diamonds of this size cost about US $500 each.

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• Motohiko Murakami, Kei Hirose, Katsuyuki Kawamura, Nagayoshi Sata, and Yasuo Ohishi: Post-Perovskite Phase Transition in MgSiO₃, Science 304 (2004) 855-858. (PDF)
• Kei Hirose, Post-perovskite Phase Transition in MgSiO₃ (PDF, from SPring-8 Research Frontiers 2004.