Kaolinite Synthesis. II. A Review and Discussion of the Factors Influencing the Rate Process

Abstract

Almtraet--Kanlinite is synthesized in approximately the same time in three temperature ranges: (1) from 200--250 ~ to 350-400* (hydrothermal processes); (2) from 120 to 175 ~ (semihydrothermal ones); (3) at ordinary temperature. It is thus evident that the rate process cannot be explained by the Arrhenius equation only, but is explained well by considering that kaolinite formation obeys the laws of crystal growth. It occurs only in slightly supersaturated solutions in which the nucleation process is possible and in which a slow and regular rate of growth has been insured. Concentrations calculated from the thermodynamicai equilibria correspond to those of the experimental conditions for the low temperature processes. For the higher temperature ones, a similar relationship is delineated, at least as far as the thermodynamical treatment can be carried out.

Morphology of Kaolinite Crystals Synthesized Under Hydrothermal Conditions

“…If S/W is high, the dissolution rate of the gels is high (Lasaga 1981), supersaturation of the solution is reached in a short time and only a few platy crystals can precipitate because of the unfavourable chemical conditions in the medium. In fact, thermodynamic considerations (Van Oosterwyck-Gastuche and La Iglesia 1978) and laboratory data (Espiau and Pedro 1984) indicated that precipitation of the hexagonal platy kaolinite is favoured from solution having a low concentration of Si and AI. At the same time, a great number of spherical particles occur in the products of the runs because their quantity does not depend on the chemical composition of the solution.…”

“…Consequently, as stated in a detailed review by Van Oosterwyck-Gastuche and La Iglesia (1978), the problem of the genesis of the mineral has been examined from many points of view. As it is expected, different physico-chemical conditions of the medium give rise to different morphological and textural appearances.…”

Influence of Synthesis pH on Kaolinite “Crystallinity” and Surface Properties

“…Despite many experiments to synthesize kaolinite at room temperature from solutions or noncrystalline materials (Van Oosterwyck-Gastuche and La Iglesia, 1978), only Espiau and Pedro (1984) obtained unquestionably a kaolin mineral after a relatively short time (three months). Thus, kaolinite syntheses were mainly performed under hydrothermal conditions (-->200~ Rayner (1962) studied the kinetic effect of temperature and synthesis duration on the rate of kaolinite formation from a (non-crystalline) gel.…”

An Occurrence of Authigenic Nacrite

“…Near perfection in crystallinity and euhedral morphology support this assumption, inasmuch as La Iglesia and Van Oosterwyck-Gastuche (1978) reported that "good" crystals form only in supersaturated solutions in which a slow and regular rate of growth is ensured. Therefore, nacrite may be an indicator of concentrated pore solutions, and the sequence of formation of kaolin polymorphs is not dependent on pH and temperature, as suggested by Gorlich (1957), but rather on the concentration of the initial solution.…”

“…Clays and Clay Minerals lutions have led to the crystallization of kaolinite and halloysite, but not nacrite (Van Oosterwyck-Gastuche and La Iglesia, 1978), suggesting, therefore, that solution concentration plays a decisive role. The absence of nacrite in the fissile shale itself demonstrates that nacrite did not precipitate in the pores of the carbonaceous shale.…”