Effect of thermal dehydration and rehydration on Na-magadiite structure.

The effect caused by dehydration and rehydration of the synthetic Na-magadiite was investigated by TGA, XRD, SEM, and (29)Si NMR. Thermal analysis of Na-magadiite presented two well-defined loss mass stages between 20 and 150 degrees C and another between 270 and 310 degrees C, both related to the removal of interlayer water. The swelling behavior of Na-magadiite was studied by thermal dehydration data obtained at 150 and 300 degrees C, and respective rehydration by water addition. X-ray patterns showed that the dehydration of Na-magadiite at 150 and 300 degrees C provoked the basal spacing decrease. The XRD also showed that only the material treated at 150 degrees C returned to the original structure with the rehydration. (29)Si NMR spectra showed that after rehydration, the Q(3)/Q(4) relationship presented the same value for Na-magadiite treated at 150 degrees C. However, this Q(3)/Q(4) value decreased when the treatment was done at 300 degrees C. Kinetic studies of thermal decomposition showed that the dehydration of magadiite is based on a phase boundary-controlled reaction, caused by contracting areas. The exfoliation of lamellas with thermal treatment can explain this behavior, as observed in SEM images.