X-ray absorption spectroscopy of mineral deposits

Abstract

X-ray absorption spectroscopy (XAS) can be used to investigate the environment of atoms or ions of selected elements in solids, which need not be crystalline, or liquids. In practice, some elements are more easy to investigate than others — for reasons which will be explained when the principles of the technique are discussed below. As a result, calcium environments in biological calcium phosphates have been extensively studied and some preliminary experiments on phosphorus environments have been performed. XAS requires a continuously tunable source of X-rays. The difficulty of obtaining sufficient intensity, over a range of wavelengths, from conventional X-ray sources was one important reason why the technique was not widely used until the availability of synchrotron radiation. Synchrotron radiation is produced by accelerators, called synchrotrons and storage rings, in which electrons or positrons move in a circular path with a typical diameter of 30 m (Catlow and Greaves, 1986). There are less than a dozen laboratories in the world with the facilities for XAS using synchrotron radiation, and, indeed, all the results described in this chapter have been obtained at three of them: LURE at Orsay (France), the Stanford Synchrotron Radiation Laboratory (USA) and the SERC Daresbury Laboratory (UK).