By applying single‐crystal X‐ray techniques, it is shown that kaolinite transforms to meta‐kaolin and subsequently to a cubic spinel‐type phase by a process of orderlyrecrystallization. The dimensional and directional relations between the phases are established. On the basis of this information, together with the density of metakaolin, a crystal structure is suggested for metakaolin. X‐ray examination of partly transformed kaolinite provides evidence for some long‐range regularity in the transformation process which is not shown by microcrystalline kaolinite.

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This reaction series has been a subject of extensive investigation since the work of Le Chatelier in 1887. Nevertheless, major problems have remained concerned with the nature of metakaolin, the manner in which it transforms into a spinel-type phase and mullite, and the relation of this spinel-type phase to mullite. The present survey brings these problems into sharp focus and provides the necessary historical background for a new approach to their solution.

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X-ray diffraction data for the high-temperature phases show that a spinel-type structure develops with marked orientation at about 925°C. This phase is considered to be an aluminum silicon spinel with vacant cation sites. Mullite is thought to be formed by the decomposition of the spinel. Silica is eliminated progressively as metakaolin transforms to the spinel phase and thence to mullite. The X-ray data show variation in the mullite parameters between 1200" and 1400°C. ; at 14OO0C., the composition probably approximates 3A1203 .2Si02. The nature of the intermediate spinel-type phase is discussed in relation to the crystal chemistry of spinels.

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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.

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