Mossbauer and infrared study of heat-treated nontronite

The Mossbauer and infrared spectra of Li- and Rb-saturated nontronites from Washington, USA (SWa-1), heat treated at different temperatures, were measured. The Mossbauer results show that diffusion of Li+ into the 2:1 layer by heat treatment at 250-300 degrees C does nor affect the magnetic hyperfine interactions of the octahedral iron centers. In contrast, significant changes were observed by calcination at temperatures >300 degrees C for Li-saturated nontronites. The main features of the Mossbauer spectra recorded at different temperatures reveal superparamagnetic behavior with a blocking temperature of similar to 5 K. The superparamagnetic behavior is related to small magnetic domains created by partially broken Fe-O-Fe bonds upon hear treatment. The infrared spectra of Li-rich nontronite, heat treated at 300 degrees C, show changes attributed to Li migration into the hexagonal cavities. Heating to higher temperatures, produced changes in the spectra of the Li- and Rb-saturated nontronites owing to the dehydroxylation of the layer structure. The dehydroxylation process begins with the loss of OH groups in the FeOHFe bridges and is completed with the disruption of the Al-O bonds at >600 degrees C.