The thermal reactions of synthetic hectorite studied by 29Si, 25Mg and 7Li magic angle spinning nuclear magnetic resonance

Abstract

The thermal decomposition sequence of synthetic hectorite (Laponite CP) is shown by a combination of thermal analysis, X-ray powder diffraction and solid-state MAS-NMR to be very similar to that of the structurally related mineral talc, in transforming smoothly into the high-temperature product without the intervention of any crystallographically distinguishable intermediates. Loss of interlayer water at about 200°C causes little change in the hectorite basal spacing or in the 29Si, 25Mg or 7Li MAS-NMR spectra; small but significant changes in the latter after heating to 600–650°C may however be related to the movement of interlayer Na + closer to the tetrahedral sheets, influencing the octahedral Li (but not the octahedral Mg). Dehydroxylation above 650°C disrupts the phyllosilicate structure into pyroxene units (MgSiC 3) and amorphous silica with distortion of the octahedral Mg sites, as evidenced by the broadening and loss of intensity of the 25Mg resonance. The 7Li MAS-NMR spectra suggest that the Li becomes mobile just prior to dehydroxylation, and may eventually be incorporated, together with the interlayer Na, in the siliceous phase.