Hydration Mechanisms and Swelling Behavior of Na-Magadiite

The hydration behavior of synthetic Na-magadiite was studied by combining thermal analyses, water adsorption gravimetry, X-ray diffraction, and infrared spectroscopy under controlled water pressure. Thermal analyses reveal, at least, two distinct water populations. The water adsorption isotherm exhibits two steps at low relative pressure. These steps correspond to three well-defined interlayer distances on the X-ray diffractograms. Infrared spectra recorded under various water pressures show distinct water populations. For relative pressures ≥ 0.20, some water molecules (1665 cm - 1 ) are doubly hydrogen bonded, likely to surface sites, whereas other (1625 cm - 1 ) exhibit a signal similar to that observed for hydrated clay minerals at low relative pressures, suggesting a strong influence of the interlayer cation. For lower relative pressures, a single water population at 1635 cm - 1 is observed, suggesting a dual influence of both the interlayer cation and the surface sites.