THE THERMAL-REACTIONS OF SYNTHETIC HECTORITE STUDIED BY SI-29, MG-25, LI-7 MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE

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-degrees-C causes little change in the hectorite basal spacing or in the Si-29, Mg-25 or Li-7 MAS-NMR spectra; small but significant changes in the latter after heating to 600-650-degrees-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-degrees-C disrupts the phyllosilicate structure into pyroxene units (MgSiO3) and amorphous silica with distortion of the octahedral Mg sites, as evidenced by the broadening and loss of intensity of the Mg-25 resonance. The Li-7 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.