Clays and clay minerals: The firing process

57Fe Moessbauer spectroscopy reveals changes in iron valence and iron site geometry when clays and clay minerals are heated, and allows a distinction to be made between paramagnetic and magnetically ordered phases. Moessbauer spectra can thus reveal the extent of iron retention in silicate structures upon heating, the identity of iron oxides initially present or formed during the heating process and their transformations, and the character of the atmosphere under which heating was carried out. This makes Moessbauer spectroscopy the most effective tool for the characterization of changes induced by heating phyllosilicates and iron oxides