Depending on the preparation temperature and the origin of the raw mineral, the products obtained by thermal activation of kaolinite lead, by hydrothermal treatment with sodium hydroxide at 102~ either to zeolite 4A, or to cubic zeolite P, or to a mixture of cubic and tetragonal zeolite P. The change in the type of zeolite obtained was explained on the basis of a radical change of the zeolite nucleation process due to either the presence of soluble potassium or high silica content resulting from dehydration of muscovite (an impurity in the raw-kaolinite), or from "segregation" associated with the first step of the metakaolinite to mullite transformation. These phenomena occur at a lower calcination temperature with poorly-crystallized kaolinite, and iron in such a sample promotes a sensitive decrease in the formation rate of zeolite crystals. Such results could be used to characterize the crystallinity of kaolinites.The products obtained by calcination or "thermal activation" of kaolinite can be used to synthesize zeolites or "molecular sieves" (Breck, 1974) by hydrothermal treatment (HT) in alkaline medium. The value of the molar Si/A1 ratio in metakaolinite (a solid more reactive than kaolinite) is equal to unity, and then corresponds to the composition of zeolite A. When metakolinite recrystallizes at high temperature giving rise to spinel, mullite or y-alumina structures, the side formation of reactive amorphous silica changed the Si/A1 ratio in the gel and theoretically other zeolites should be obtained, e.g. faujasite (Flank, 1970).The aims of the present work were two-fold.(i) to verify the existence of a thermal activation temperature (Tc) of kaolinite giving rise to a maximum of chemical reactivity versus sodium hydroxide solution in hydrothermal conditions (synthesis of zeolite A). Effectively, in previous papers, it was shown that some parameters (X) directly connected with both structural disorganization and chemical reactivity of metakaolinite (Murat et al., 1989), e.g. "short range disorder" (Cd) measured by infrared spectroscopy (Bachiorrini & Murat, 1986), "disorganization factor" (Fd) measured from dissolution kinetics in dilute hydrofluoric acid (Murat & Driouche, 1988), and "dissolution enthalpy" (]AHd]) in concentrated HF (Murat et al., 1987), present a maximal value with metakaolinite sample prepared by calcination of kaolinite at Tc ~-720-750~ The same result was obtained concerning the "pozzolanic reactivity" of metakaolinite (same behaviour as "pozzolanas", e.g. amorphous Italian volcanic ashes, 9 1992 The Mineralogical Society

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