ALUMINUM COORDINATION AND STRUCTURAL DISORDER IN HALLOYSITE AND KAOLINITE BY AL-27 NMR-SPECTROSCOPY

It has been suggested that interlayer waterin halloysite is due to the presence of hydrated cations that balance the negative layer charge p roduced by Al for Si substitution. To find evidence of 4-coordinate Al (Al(IV)), we investigated six halloysites and two kaolinites using 'h igh-field' and 'medium-field' solid-state 27 Al MAS NMR spectroscopy. We found Al(IV) in both kaolinite and five halloysite samples, but the contents are all <1% and provide no basis for distinguishing between kaolinite and halloysite. Therefore, the presence of interlayer water in halloysite cannot be attributed to Al for Si substitution. There ar e, however, signals, tentatively assigned to Al(V), present in the kao linite spectra but not in the halloysite spectra. The shapes of the lo w-frequency 'tails' of AI(VI) signals in medium-field NMR vary from sa mple to sample. We interpret this variation in terms of a 'crystallini ty index'. Disorder in kaolinite appears to be primarily the result of Al-vacancy displacements in the octahedral sheet. The NMR crystallini ty indices correlate with those from IRS and DTA but not with those fr om XRD.