Explainer: What is crystallography

31 Oct 2012, 11:00 am   -   Source: The Conversation
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Crystallography has been instrumental in 26 Nobel prizes, including the last four chemistry prizes. (Getty)

Crystallography has been instrumental in 26 Nobel prizes, including the last four chemistry prizes. (Getty)

When NASA's Curiosity rover scooped soil into its belly, this simple action proved to be crucial for a lesser-known branch of science called crystallography, which is celebrating its centenary this month.

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By Helen Maynard-Casely, Australian Synchrotron

About a week or so ago the Mars Science Laboratory, Curiosity, literally swallowed some dust. Scooped from the soil about the rover, its robotic arm manoeuvred to a hole and tipped the dust into the belly of the rover.

‘Down the hatch’ Where the martian dust was tipped for delivery to the ChemMin instrument. NASA/JPL

There’s a very particular group of scientists that were very excited about this development. You won’t see them at mission control, sporting outrageous hair do’s and jumping wildly (which is a shame) but the people behind ChemMin were probably very happy and relieved to see this scoop of dust being swallowed and sent to their instrument in Curiosity’s belly. They’d had to wait since the 6th of August to verify if their instrument was even working after its long trip to Mars and eventful landing, a long three months. On top of that what they wanted to do was pretty audacious, carrying out the first crystallography on another planet.

Crystallographers are perhaps not the most well known group of scientists. The title has people wondering if they are, in fact, professors at Hogwarts. But crystallography is the ‘science of the arrangement of atoms in solids’, and is spectacularly useful for a massive range of sciences. And now, on the eve of its centenary the field has taken another giant leap into the future.

It’s coming up to a century since, on November 11th, 1912 a young man presented the first crystal structure solution to the Cambridge Philosophical Society. In the proceedings that were published after this event William Lawrence Bragg described how he used a set up of an X-ray tube and an image plate, similar to that carried on Curiosity, to work out the arrangement of atoms in the material zinc blende. His ‘Eureka’ moment in this discovery was to think of the X-rays travelling through the zinc blende as waves, allowing him to interpret the spots he got in the images plate as reflections.

When you throw two stones into a pond, the ripples from each stone interact with each other. What Bragg was observing on the image plate was the result of ripples from the layers of atoms in the zinc blende structure. These realisations allow him to interpret the spots he observed, and work out where the each atom within zinc blende was.

Adelaide-born Bragg was, along with his Father William Henry Bragg, awarded the Nobel Prize for physics in 1915 for this work. At 25 he was, and still is the youngest recipient of the award. He didn’t stop there, and went on to establish and put down much of the groundwork of the science and practise of crystallography. You only have to look at subsequent Nobel prizes to see the impact this has had. Crystallography has been instrumental in another 25 Nobel prizes, including the last four chemistry prizes.

The first x-ray diffraction pattern from another planet! NASA/JPL-Caltech/Ames

So somewhere out there a lucky crystallographer is examining the first of these reflection patterns taken on another planet. The raw data has just been released; it’s beautiful but without detailed analysis is impossible to make direct conclusions from these data. Unlike ChemCam, where the raw data peaks will correspond to a particular bond between a couple of atoms, these images of the reflections (more often called diffraction patterns) are more devilish to interpret.

As Bragg discovered a hundred years ago, the peaks in a diffraction pattern (which are often called Bragg peaks) correspond to the spacing between atoms in crystals. To see if a particular material is present needs all the peaks to be there, and then to fit a model of your crystal structure to the data. So it’s tricky, and you have to be careful to check that all the information you want is there.

In yesterday’s press release, the ChemMin scientists revealed that they are pretty sure the sample contains olivine, making the dust similar to the weathered dust that you can find on a Hawaiian beach, product from a volcano. So far the scientist have commented that the results are supporting their original idea that Curiosity’s landing site, Gale Crater, has changed from a wet to a dry environment in Mars’s history. But much remains to be picked out of the patterns, with each material they find capturing a little of Mars’s geological history. Like Bragg one hundred years ago, the interesting thing will be when these, now Martian crystallographers, succeed in putting all the pieces together.

Helen Maynard-Casely does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

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