Saturday, June 05, 2010

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".

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.


Phil Warnell said...

Hi Stefan,

It’s been some time since you’re last posting and I was beginning to wonder where you went. I must say I found this article very informative as I don’t know as much as I’d like about what’s going on in the earth sciences. To begin with I wasn’t aware the earth’s mantle is primarily composed of silicates with the prime one being magnesium silicate.

It does seem rather ironic that one would use the hardest natural substance to put the squeeze on one of the softest, as the most common form of magnesium silicate being talc. This has me to imagine earth’s interior in a whole new way. Also as this mineral on the surface is never found in large crystalline form how such a substance would physically present itself leaves me curious. You also seem to suggest in this post that perhaps this character is only present as long as the pressure is maintained.

Anyway it’s surprising to learn that something that’s best known for keeping a baby’s bottom dry to be so important in understanding our planet’s interior:-)



P.S. I do remember reading once that the core of Jupiter is speculated to be diamond, and so much for its intrinsic value in being so rare, as actually it relates more to acccessabilty:-)

Uncle Al said...

It is beautiful that the gasket (rhenium?) is always under more pressure than the sample, maintaining the seal. What is the pressure transfer medium?

Bee said...

I just love it that they wrecked the diamonds with a hand screw. It's amazing what pressure one can build up if one focuses a force sufficiently!

Georg said...

P.S. I do remember reading once that the core of Jupiter is speculated to be diamond, and so much for its intrinsic value in being so rare, as actually it relates more to acccessabilty:-)

Hello Phil,
Even in the Region of this CMB
all carbon will exist as diamonds.
In Jupiters core -(the appropriate
place for speculation this days,
because the earths core now is
within reach of science)-
The pressure will be much higher, and it is rather likely that
carbon will be in a metallic
state (closest packing).

Steven Colyer said...

What's up, Stefan? I echo Phil's comment of long time, no hear, and don't be such a stranger. Actually, I don't think it was all that long ago Phil, it just seems so in this Information Overload age, in which the perception of time is compressed.

Stefan, I like this diversion of yours into Earth Science, well done. D-double prime, heh. Who knew? Cool. It never ceases to amaze that we know more about the surface of the moon than what goes on inside our own planet, let alone our oceans (now with more petrochemicals).

So what's up Stefan? If I recall correctly you were preparing for your doctoral dissertation in QCD. Is that right and if so when is the deadline?

Note to the young-uns: Please do not see the film "The Core" for anything even remotely resembling real science. See it as entertainment, only.

stefan said...

Hi Uncle Al,

It is beautiful that the gasket (rhenium?) is always under more pressure than the sample, maintaining the seal.

Good point - that didn't occur to me, but of course, you do not want the compressed stuff to squeeze out sideways ;-). You are right, the gasket is made of rhenium.

What is the pressure transfer medium?

Pressure transfer into the sample? Or pressure for keeping in place and working the gasket? Sorry, I have no idea - all I know about the experimental details is from the Science paper (PDF here) and this longer Review paper (PDF here) by Kei Hirose. In the Science paper, they write:

... MgSiO3 gel was used as a starting material. It was mixed with platinum powder that served both as an internal pressure standard and a laser absorber. The sample mixture (∼25 µm thick) was loaded into a 60-µm hole drilled in the rhenium gasket together with insulation layers of MgSiO3 gel unmixed with platinum (∼10 µm thick on both sides).

I was impressed especially about the clever use of platinum, as both absorber of light for heating and as a means of monitoring the pressure achieved via the known dependence on pressure of the lattice parameters...

But maybe one of our readers knows more about such experiments and can comment?

Cheers, Stefan.

Phil Warnell said...

Hi Stephan,

Just picking up on the point Bee made as to marvel at how something so small can be used to create such tremendous pressures had me reminded this device puts into practice the discoveries of two renown scientists from the past. The first being Archimedes with the realization of the power inherent in levers and screws and secondly with Galileo having us understand that things become proportionally stronger as they are scaled down, instead of decreasing or remaining the same, which would seem more intuitive. It’s nice to know that older discoveries still stand as being useful in respect to the modern inquires of science.



stefan said...

Hi Phil,

right, talc is also a magnesium silicate, albeit with some OH groups included in the crystal lattice...

Actually, it seems that magnesium silicates can come in a variety of specific chemical compositions and crystal structures, all resulting in different minerals - that's quite a zoo ;-).

The special thing about the magnesium silicate in the mantle is the perovskite structure (and post-perovskite all down...), but I am not sure if the perovskite modification is stable under "normal" conditions (any mineralogist around?). Anyway, the post-perovskite turns back into perovskite if pressure is released.

Cheers, Stefan

stefan said...

Dear Bee, Phil,

... that they wrecked the diamonds with a hand screw.

Amazing indeed :-). The central idea of the experimental setup is so simple! But well, the practical realization obviously isn't....

Cheers, Stefan

Phil Warnell said...

Hi Stefan,

Yes magnesium silicates can come in many forms other than talc, with one of those being the now dreaded asbestos. From what I’m able to pick off the web the perovskite form of it was not discovered until about ten years ago in earlier experiments like the one you describe here. This has me to think that perhaps it can only exist at such pressures, although I can’t find anything that directly confirms or denies this.

I find this interesting as it has one reminded how it is not only life that depends on special conditions to exist, yet more rudimentary things as well. All this talk about layers of the earth also had me reminded what a small portion of it we are restricted to, with being between the crust and the atmosphere, which themselves would be comparatively each less than the thickness of a skin on an apple.

Anyway as it turns out this perovskit form of magnesium silicate is actually the most abundant mineral that makes up our planet and yet until a few years ago we never knew it existed is quite a revelation for me. With all the fascination with what has the universe be as it is, with having us peer into the heavens, it’s things like this that serve to remind there are still things to be discovered laying just beneath our feet.



Uncle Al said...

Face-centered cubic diamond is space group Fd3m - not a particulary dense lattice arrangement of atoms. Silicon and germanium, both in Fd3m, expand when their liquids freeze.

Diamond is remarkably fragile toward [111] cleavage. A well-oriented tap will spall off triangles with hardly any effort. A skilled diamond cutter orients facetting in directions that will not be bumped off. Diamond anvils must be oriented for strength.

Plato said...

Secrets of the Pyramids In a boon for archaeology, particle physicists plan to probe ancient structures for tombs and other hidden chambers. The key to the technology is the muon, a cousin of the electron that rains harmlessly from the sky.

Of course you raise insights into my mind Stefan about the mining on the moon and the area I have claimed.:)

The use of collision processes to uplift particulates into the moon atmosphere for dissection, had me reflect on Newton's own alchemical nature of investigation into the construct of mineral lines in our planet earth.

A Hollow Earth! Naw!

So historically such thinking advanced to the current issues in your article bring us into the advances of science as we get that new view of earth.

So a question came to mind.

How does the mineralogical data affect our views of the interior of the earth?

Geo-neutrinos are anti-neutrinos produced in radioactive decays of naturally occurring Uranium, Thorium, Potassium, and Rubidium. Decays from these radioactive elements are believed to contribute a significant but unknown fraction of the heat generated inside our planet. The heat generates convective movements in the Earth's mantle that influence volcanic activity and tectonic plate movements inducing seismic activity, and the geo-dynamo that creates the Earth's magnetic field.