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°C, 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.