INTERACTION OF KAOL IN ITE WITH CALCITE ON HEAT ING: I I I. EFFECT OF D IFFERENT KAOL IN ITES

ABSTRACT: The degree of distortion of the calcite endotherm on differential thermal analysis (DTA) curves for kaolinite-calcite mixtures depends on the kaolinite sample used as well as on the factors previously established. Although no two of the ten kaolinite samples employed, even if from the same general ocality, produced identical effects under all experimental conditions, a general relationship between locality of origin and degree of distortion was noted. The crystalline species detected inthe products of heating, after standing in air at room temperature, included portlandite, calcite, aragonite, vaterite and the high-temperature phases gehlenite, c(-Ca2SiO, and 12CaO. 7A1203. The actual species present again depended on the kaolinite sample. Although the results cannot be directly related to the volatiles evolved along with water on dehydroxylation f the different kaolinites, atentative explanation is offered on the basis of the effects of the volatiles on the surfaces of the particles present. Earlier studies (Mackenzie & Rahman, 1987; Mackenzie et al., 1988) have established that the configuration of the calcite endotherm on DTA curves for kaolinite-calcite mixtures is highly dependent on instrumental nd procedural variables, whether the atmosphere be air, nitrogen or carbon dioxide. The complexities were not due to solid-state reactions and wer