X-ray absorption spectroscopy in mineralogy: Theory and experiment in the XANES region
X-ray absorption spectroscopy has become a common technique in mineral studies only in fairly recent times. It is an element-specific method which is suited to extend structure determination down to the local environment of an atom, i.e. a volume some three orders of magnitude less than that inspected by methods based on X-ray diffraction. However, in line with many other modern techniques, X-ray absorption spectroscopy is neither simple as for the practical operations by which one records high-quality experimental results, nor it is straightforward in the interpretation of them, the more so as minerals are far more complex multi-atomic systems than most compounds investigated by other material scientists. Consequently the mineralogical literature related to X-ray absorption spectroscopy is full of misunderstandings, which may even become traps for a new user. A further motive for the poor interpretation of experimental results that are otherwise technically excellent arises from the bare fact that the theoretical framework of X-ray absorption spectroscopy lies well beyond the basic physics normally taught to mineral and material science students. Indeed, this is possibly why quite a few people have used this powerful technique as if it were a black box (e.g. the ominous “fingerprinting” practice!), or they have overextended the interpretation of spectra beyond what is their true potential content (cf. Stern, 2001).
In this chapter, I try to show all what is possible as well as all what is reasonable to obtain by the main absorption spectroscopy methods in use at the present time
Figures & Tables
Spectroscopic methods provide information about the local structure of minerals. The methods do not depend on long-range periodicity or crystallinity. The geometric arrangement of atoms in a mineral phase is only one aspect of its constitution. Its vibrational characteristic, electronic structure and magnetic properties are of greatest importance when we consider the behaviour of minerals in dynamic processes. The characterisation of the structural and physico-chemical properties of a mineral requires the application of several complementary spectroscopic techniques. However, it is one of the main aims of this School to demonstrate that different spectroscopic methods work on the same basic principles. Spectroscopic techniques represent an extremely rapidly evolving area of mineralogy and many recent research efforts are similar to those in materials science, solid state physics and chemistry. Applications to different materials of geoscientific relevance have expanded by the development of microspectroscopic techniques and by in situ measurements at low- to high-temperature and high-pressure conditions.