Publisher Summary This chapter describes X-ray absorption spectroscopy (XAS). XAS corresponds to a range of X-ray spectroscopic techniques based on scattering, absorption, and emission (fluorescence, auger) of X-rays by matter. Synchrotron-based X-ray sources produce an intense X-ray emission with total fluxes and flux densities several orders of magnitude greater than conventional X-ray tubes. As a result, a nearly flat spectral distribution is produced over a large range of mineralogically important elements. This and recent advances in X-ray optics, detectors, insertion, and focusing devices as well as modeling of scattering processes made XAS a valuable tool for obtaining atomic-level information not previously attainable by conventional X-ray sources. The chapter reviews synchrotron-based studies of clays and clay minerals, focusing on the application of XAS to elucidating the structure and reactivity of 2:1 phyllosilicates, with particular emphasis on smectites.
