ALUMIN IUM COORDINAT ION AND STRUCTURAL DISORDER IN HALLOYS ITE AND KAOL IN ITE BY 27AL NMR SPECTROSCOPY

A B S T R A C T: It has been suggested that interlayer water in halloysite is due to the presence of hydrated cations that balance the negative layer charge produced by AI for Si substitution. To find evidence of 4-coordinate AI (AI(IV)), we investigated six halloysites and two kaolinites using 'high-field ' and 'medium-field ' solid-state 27A1 MAS NMR spectroscopy. We found AI(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 AI for Si substitution. There are, however, signals, tentatively assigned to AI(V), present in the kaolinite spectra but not in the halloysite spectra. The shapes of the low-frequency 'tails ' of AI(VI) signals in medium-field NMR vary from sample to sample. We interpret his variation in terms of a 'crystallinity index'. Disorder in kaolinite appears to be primarily the result of AI-vacancy displacements in the octahedral sheet. The NMR crystaUinity indices correlate with those from IRS and DTA but not with those from XRD. The presence, or history, of a sheet of interlayer water provides the essential distinction between halloysite and kaolinite (Churchman & Carr, 1975; Bailey, 1990). On the basis of a critica