Kinetics of Dehydroxylation of Kaolinite and Halloysite

Previous studies of the kinetics of dehydroxylation of kaolinite and halloysite point to first‐order reactions, in approximate conformity with the Arrhenius relation. Isothermal weight‐loss measurements have shown that the rate constants are markedly dependent on factors such as specimen size, shape, and compaction. A technique has been developed for determining the reaction kinetics of infinitely thin disk‐type specimens. The reactions are then strictly first order and the Arrhenius relation is obeyed. Activation energies of 65 and 55 kcal. per mole are obtained for kaolinite and halloysite, respectively. Comparison is made between the behavior of kaolinite and halloysite on the one hand and of macrocrystalline anauxite on the other. For anauxite, nucleation and growth of nuclei produce a sigmoid‐type reaction curve, but for the fine‐grained minerals it is believed that nucleation alone is the rate‐controlling process. The dependence of the reaction rate on geometrical factors is attributed to the retention of water vapor within the powder specimen. The influence of water vapor on these reactions is discussed generally.