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Thermal transformations of kaolinite were studied by zgsi-and ZVA1-MAS/NMR and ESR techniques. In metakaolin, Si is still dominantly in the Q3 state (three Si atoms bonded to an Si-O4 tetrahedron) and A1 detectable by NMR is in both 4-coordination and 6-coordination. Coordination polyhedra around A1 or Fe replacing A1 are much distorted. Metakaolin crystallizes into y-alumina and mullite exothermically at ~ 980 ~ this crystallization being preceded by a faint endothermic reaction. The latter is due to segregation of SiO2 and A1203, which results in an increase of Si in the Q4 state and an increase of 6-coordinated A1203 .Kaolin minerals dehydroxylate at ~600~ this process corresponding to an intense endothermic peak on the DTA curve. The resultant anhydrous phase, called metakaolin or metakaolinite, is amorphous to X-rays. y-alumina plus mullite develop from the amorphous phase at ~ 980~ and this reaction is represented by a combination of a faint endotherm and a sharp and intense exotherm. The mechanism of this series of reactions has been studied from various standpoints, as reviewed by Brindley & Nakahira (1959), Tsuzuki (1971) andBrindley (1976). X-ray diffraction, electron microscopy, and X-ray fluorescence, IR absorption and M6ssbauer spectroscopy have all been used to study these reactions. Recent developments in high-resolution magic-angle-spinning MAS/NMR spectroscopy have allowed a new approach to this problem, Komarneni et al. (1985), MacKenzie et al. (1985b) and Brown et al. (1985) being forerunners in the application of this method to metakaolin and high-temperature phases of kaolinite. This paper gives the results of examination of a kaolinite heated to various temperatures, using 29Si-and 27A1-MAS/NMR and ESR techniques. Some new data are presented on the thermal transformation of kaolinite.

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XRD, DTA, IR, chemical and electron microscopic data are presented for three specimens of regularly interstratified 25 A minerals from Roseki deposits in Japan. The layer charges of these dioctahedral mica-smectites originate mainly from tetrahedral Al-for-Si substitution. The non-expandable interlayers are occupied by relatively large amounts of Ca and K as well as Na. The expansion characteristics of the expandable interlayers are montmorillonitic in spite of the beidellitic composition of the 2:1 layers.