Mössbauer 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 57Fe Mössbauer spectroscopy is among the most straightforward to operate. 57Fe Mössbauer 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, Mössbauer 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 Mössbauer spectroscopy is presented, followed by examples of Mössbauer spectra of minerals that are common constituents of clays and soils, and by more complex cases of soils, clays and fired clays.