Clays and clay minerals: What can Mossbauer spectroscopy do to help understand them?

Mossbauer spectroscopy is a powerful technique for the characterization of materials formed in the weathering environment. Mossbauer studies of clay-s ized phyllosilicates, however, are burdened with several problems: the samp les are rarely monomineralic, they may be poor in iron, and only few iron-r ich species order magnetically above 4.2 K. Site occupancies are difficult to determine, and cis and trans octahedral-OH site assignments are normally not possible. Unequivocal information that can be gained from such work th us is often restricted to the determination of the oxidation state of iron and average structural site distortions. Mossbauer data on iron oxides are generally more straightforward to interpr et because these can be studied in the magnetically ordered state. A furthe r asset of Mossbauer spectroscopy when studying iron oxides lies in its hig h sensitivity for magnetically ordered phases. Adverse effects ensuing from small particle size, interparticle interactions, nonstoichiometry and fore ign-element substitution that often affect the Mossbauer parameters of iron oxides occurring in clays and soils can be at least partly offset by takin g spectra at low temperatures.