By Penny Carmichael, on 21 February 2014
– Article by Jack Humphrey
UCL’s own Professor Paul McMillan takes us on a journey to the centre of the earth in search of the weird and wonderful chemistry that occurs at high temperatures and pressure with a smattering of biology too.
Everything you know about chemistry is only true under specific temperatures and pressures. We do our chemistry on the outer surface of the earth’s crust, where the temperatures vary in the range of a few hundreds of degrees and pressure is around 1 atmosphere which is 100 kPa. Here the periodic table works rather nicely and the fits with the observations we can make. But what happens if we go deeper? The largest hole ever drilled goes down about 12km, not even breaking through the crust into the mantle. If we could somehow reach the Earth’s core we could do experiments at 7000°C and at a pressure of 360 GPa.
And why should we care about this? To understand planets. For example, the centre of Neptune has been worked out to be at around 800 GPa due to its large size. And as we know it’s composed of methane, water and ammonia, what do pressures as ludicrously large as that do to the chemistry of these molecules? How do they interact with each other? Professor McMillan’s lab is interested in the weird chemistry of high pressures and temperatures. They do this using the not-so-humble diamond.
Diamonds are thrown up from the mantle in places like South Africa where the earth’s crust is very old. Diamonds are formed from graphite that is put under the high temperatures and pressures found 120 km below the surface. They are forced upwards by pressurised gases and mechanical weaknesses in the rock to where they can be mined. But what if we could make them ourselves? The artificial diamond was created thanks to advances in pressure seals made by Percy Bridgman, who won the 1946 Nobel for his work. These high pressures (around 25 GPa) allowed General Electric to create the world’s first artificial diamonds and more have now been produced in a high throughput process than have ever been mined. But what about pressures higher than 25 GPa? Let’s throw diamonds at the problem. By putting two flat ended diamonds together in a vice, whatever is between them is crushed with a pressure of ~400 GPa. And thanks to the transparency of diamonds, you can heat your samples up with a laser!
The diamond anvil press has been used to investigate the changes in rock structure that occur with depth into the earth. Silicates make up the majority of rocks on earth and exist in a 4-coordination state at the upper mantle but are compressed into a spinel form as it goes further down into the mantle and then finally a 6-coordination state before it reaches the core. A current debate is raging in high pressure chemistry over the state of hydrogen under high pressures. It’s possible to grow crystals of hydrogen, modelling the high pressures inside Jupiter. But it’s controversial whether or not this crystalline hydrogen is metallic or not, with some labs showing conductivity under shockwave pressure (using essentially a battering ram) but this hasn’t been replicated in the diamond anvil. However, elements like Iodine do become metallic at 20 GPa and superconducting too. It would in fact appear that all elements are superconductive above 100 GPa.
And what about biology? The bottom of the Mariana Trench, 10 km below sea level, is at around 100 MPa and is teeming with life. These are piezophiles, incredibly hardy organisms that have adapted to extreme pressures. But can we recreate this in the lab? Professor McMillan has created piezophilic E. Coli by culturing them in the diamond anvil and gradually increasing the pressure. This form of natural selection is a model of adaptation driven by an outside stressor, another of which is antibiotic resistance. Hopefully the mechanisms discovered in these pressure experiments will be useful in the fight against antibiotic resistant pathogens.