Mossbauer spectroscopy of clays, soils and their mineral constituents

Mossbauer spectroscopy is the technique of recoil-free resonant emission and absorption of gamma rays. It has the advantage of being oblivious to all elements except the one under survey. In the case of materials formed on the earth's surface, such as soils and clays, the only propitious element is Fe. Iron is the fourth most abundant element in the Earth's crust, it is essential for life, and almost all environmental materials contain at least some Fe. It is also fortuitous that Fe-57 Mossbauer spectroscopy is among the most straightforward to operate. Fe-57 Mossbauer spectroscopy thus allows the characterization of iron speciation, and thereby of environmental conditions, over a wide range of concentrations, making it an extremely effective environmental probe. Straightforward as it may seem, Mossbauer spectroscopy nevertheless has many pitfalls. Besides problems arising from the basic physics, complications can arise among other causes from imperfect crystallinity (small particle size), non-stoichiometry, interparticle effects and isomorphous substitutions. In this paper a succinct review of the basic principles of Mossbauer spectroscopy is presented, followed by examples of Mossbauer spectra of minerals that are common constituents of clays and soils, and by more complex cases of soils, clays and fired clays.