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A detailed characterization of elements within clay minerals and soils usually requires the use of a number of different techniques. Spectroscopic methods are often (and should be) incorporated in the characterization procedure as they provide a wealth of information on the chemical and structural nature of an element within solids. A spectroscopy that has recently proven to be a powerful means for obtaining the speciation and local structure of elements present in clay minerals and soils is x-ray absorption fine structure (XAFS) spectroscopy-the subject of this chapter. XAFS has a number of advantageous qualities for studying clays and soils which include: element specificity, the local chemical and structural state of an element, and the ability to analyze materials in situ. It probes the local chemistry and structure of a single element throughout a sample. What is captured by this technique can be thought of as a ‘view’ of the x-ray absorber’s electronic structure and the atoms that coordinate it; Figure 1 illustrates the structural ‘view’ obtained with this method. The oxidation state, type of nearest neighbors, coordination number, bond distances, and orbital symmetries of the x-ray absorbing element can be accurately determined in an array of media (Eisenberger and Lengeler 1980). Because the information obtained with XAFS differs from that of other spectroscopies and microscopies, when used in conjunction with them XAFS offers a complementary means for detailing the properties of clay minerals or soils.
Basic, general steps for performing XAFS and a brief background on its physical basis are