THE REHYDRATION OF METAKAOLINITE TO KAOLINITE - EVIDENCE FROM SOLID-STATE NMR AND COGNATE TECHNIQUES

We have studied the process of rehydroxylation of metakaolinite to kaolinite by 27Al and 29Si MAS NMR in tandem with Fourier transform infrared spectroscopy, powder X-ray diffraction, thermogravimetric analysis, specific surface area measurement, atomic absorption spectrophotometry, and electron microscopy. The efficiency of the process strongly depends on temperature. The reaction is initiated at the edges of metakaolinite particles and is followed by diffusion of the water into their bulk. Regions of the metakaolinite structure, which locally retain the symmetry of kaolinite, act as nuclei for the reconstitution of kaolinite particles. The latter grow perpendicular to the c* axis and their edges are parallel to those of metakaolinite, showing that rehydroxylation is a topotactic solid-state process. 27Al and 29Si MAS NMR reveal that metakaolinite is an ill-defined mixture of amorphous silica and alumina, and can be transformed into kaolinite by a correct choice of experimental conditions. NMR also shows that: (i) the 5-coordinated Al occupancy decreases with the reaction time faster than that of 4-coordinated Al, which supports the view that 5-coordinated Al in metakaolinite acts as a structural defect; and (ii) the 001 and 002 XRD reflections of kaolinite are only observed when the 5-coordinated 27Al signal is faint. Further studies on the synthesis of kaolin (and indeed other clay minerals) are needed to assess the role of 5-coordinated Al in the process. © 1990.