Behavior of paramagnetic iron during the thermal transformations of kaolinite

Kaolinites with various degrees of structural order and iron content were h eated and subsequently analyzed via electron paramagnetic resonance. Iron w as present in two different states in the heated materials, either as dilut e structural Fe3+ ions or in concentrated Fe3+ phases. During metakaoliniza tion, the environment of dilute Fe3+ ions changed, following modifications of the Al3+ coordination, and the Fe3+ concentration increased. With the br eakdown of metakaolinite, the diffusion of Fe3+ ions induced their exsoluti on in superparamagnetic iron-rich domains (Fe3+ clusters in gamma -Al2O3 an d/or Fe3+ oxide nanophases), which produced a decrease in the dilute Fe3+ c oncentration. The subsequent breakdown of gamma -Al2O3 and the formation of mullite made the dilute Fe3+ concentration increase again, because of the incorporation of Fe3+ ions in the mullite structure.