This year, 2014, celebrates ‘the 100th anniversary of X-ray crystallography, a technique that I regularly use in my research on the structure of minerals. The centennial marks the first determination, in 1914, of an atomic structure. William Henry Bragg and his son William Lawrence solved the crystal structure of a very common mineral, halite, which is sodium chloride. They were the first to derive the law of physics that governs the relationship between X-ray diffraction and the arrangement of atoms in a crystal—now known as Bragg’s Law.
During the 38 years I have worked at the Canadian Museum of Nature, I have solved over 100 crystal structures and described about 100 new minerals. Much of the work done in the museum’s X-ray lab is for routine mineral identification.
Minerals are notoriously difficult to identify as many can have very similar physical properties, yet their ‘invisible’ properties, such as chemical composition and atomic structure, are vastly different. Knowing exactly what mineral you have is essential to the mining industry and to geological interpretation.
I have used analyses of crystal structure in a number of ways. They have helped describe new mineral species, such as moydite, named after Lou Moyd, the museum’s first curator of minerals. Moydite was investigated by other researchers as a possible containment material for radioactive cesium, a dangerous by-product from nuclear-power generation. More recently, I have also been investigating structure changes in minerals such as veatchite and hilgardite to help interpret the geological conditions in salt formations such as the potash deposit in Sussex, New Brunswick.
Now, the word X-ray is most familiar to people in terms of medical and dental uses, radiography, and airport security imaging. These X-rays have a much shorter wavelength and higher energy than those used in crystallography; hence, their ability to pass through materials such as human tissue or suitcases to a detector that displays the transmitted image.
For crystal-structure determination it is necessary to choose an X-ray wavelength comparable in size to that of an atom. Shooting X-rays into a crystal creates the phenomenon of diffraction off planes of atoms. This is known as ‘Bragg diffraction or reflection’. The resulting image appears as a set of dots or reflection peaks.
The intensity of a peak is proportional to the total number of electrons associated with atoms in a plane. As an example, planes of atoms that contain lead, which have a lot of electrons, diffract an intense peak while planes composed of atoms of oxygen, with much fewer electrons, produce a much less intense peak.
It was mineralogists who established early developments in X-ray crystallography, since they had access to crystals grown in nature. Today the vast majority of crystallographers are in the field of inorganic chemistry studying proteins, DNA and drugs. To conduct their structure-determination experiments they must first grow a crystal. Crystal growth is as much a science as it is an art but without a crystal no X-ray diffraction is possible. A single molecule does not diffract X-rays. Once the crystal is produced the important work to determine the crystal structure begins—and it all started 100 years ago with the determination and insight of the Braggs.