Coexistence States of Iron in Synthesized Iron-bearing Allophane (Al2O3-SiO2-Fe2O3-H2O System)

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A sharp exothermic peak at 980°C, which is found characteristic of allophane in its differential thermal analysis, shifted to the lower-temperature side and broadened as the iron content in the iron-coprecipitated samples increased. In the case of the iron-mixed samples, however, this exothermic peak did not shift, and a new broad exothermic peak, which was ascribed to hydrated ferric oxide, appeared. The crystallization processes in the heat-treated samples were found to depend on the coexistence state and the quantity of the iron. The dimensions of a unit cell of the mullite, crystallized at 1200°C from the iron-coprecipitated samples, varied with the iron content. The iron-mixed samples, however, showed little change in their cell dimensions. It is concluded that there are two states of the coexistence of iron in the allophanes. In one of them, iron ions form solid solutions of the allophane, i.e., as an iron-allophane. In the other, iron is in the mixture as the hydrated ferric oxide. The maximum amount of iron in the solid solution was estimated to be 5 wt% in the iron-allophane.
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