X-ray absorption spectroscopy in geosciences: Information from the EXAFS region
Although absorption of X-rays by matter has been observed since a long time, it is the recent availability of synchrotron radiation sources that has established EXAFS spectroscopy as an important structural tool in geosciences. Nowadays, X-ray Absorption Fine Structure (XAFS, including both the EXAFS and XANES domains) has proven to be an unquestionably useful and powerful method to obtain information on the local and medium-range structural order around a chemical element in condensed matter (solids, liquids, interfaces etc.). This chapter provides basic information on EXAFS (Extended X-ray Absorption Fine Structure), including some recent experimental developments, both on data acquisition and on spectra reduction, and will be illustrated using recent applications in mineralogy, geochemistry, materials science and environmental science. XANES is separately discussed by Mottana (2004) in this volume.
The experimental and theoretical details of extended X-ray absorption fine structure, EXAFS, have been extensively described in the literature (e.g. Teo, 1986; Sayers & Bunker, 1987; Koningsberger & Prins, 1988; Lytle, 1989; Mustre de Leon et al., 1991; Lytle, 1999). Recently, the technique has gained wide popularity, due to major breakthroughs both in EXAFS theory and data analysis with (i) the availability of convenient software packages (www.esrf.fr/computing/scientific/exafs/links.html) and (ii) the development of the use of the ab initio multiple scattering code FEFF (Rehr & Albers, 1990; Zabinsky et al., 1995; Ankudinov et al., 1998; Ankudinov & Rehr, 2003).
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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.