29Si MASNMR investigations of the thermochemistry of laponite and hectorite

Abstract

29Si MASNMR spectra were used (together with some 23Na MASNMR spectra and XRD and IR measurements) to monitor the thermolysis of laponite-RD and hectorite. For laponite, even at 400°C, some breakdown of the silicate ( Q 3) lattice is observed although this is reversible on rehydration. Lithium ions migrate from trioctahedral positions at 600°C and by 800°C irreversible breakdown to polymorphs of enstatite occurs. At 1300°C ortho- and clino-enstatite co-exists with a glassy phase. The thermal behaviour of hectorite is broadly similar. 23Na MASNMR spectra reveal that the location of sodium ions in rehydrated specimens, particularly for laponite heated to 400 and 600°C, may be very “solution”-like. The exchange-doping of hectorite with paramagnetic ions such as Cu 2+, Ni 2+, Mn 2+ was explored; the loading level beyond which resolution of the silicon spectra is lost was defined in terms of gramme magnetic susceptibilities. The combined use of 29Si MASNMR and ESR to monitor the thermolysis to 1200°C of a sample of hectorite with a Cu 2+ loading of 10.5 mg g −1 clay is illustrated.