Thermal transformation of goethite into haematite in alkali halide discs

Parallel thermal treatments have been investigated with powder mixtures containing 33 wt % goethite in alkali halides (KCl, KBr, KI and CsI) and with pressed discs having the same compositions. Infrared and X-ray studies have shown that the reaction scheme occurring in the discs includes: (1) dehydroxylation of goethite leading to the formation of protohaematite (at 250°C) and (2) recrystallization of protohaematite into haematite (at 430°C). On the other hand, only the dehydroxylation stage occurs in the powder mixtures and protohaematite persists even after prolonged heating at 430°C. The recrystallization stage, which is accompanied by crystal growth, is very slow in the CsI discs, whereas crystal growth by aggregation (sintering) is essential in the CsI disc but not in the KCl, KBr and KI discs. The results of weight loss during the thermal treatment indicate that in addition to dehydroxylation other reactions are taking place, mainly in the discs. These reactions may include ion exchange and reduction of ferric iron. Water adsorption of the various salt-free products was studied in three different environments, namely from liquid water and from 100 and 40% relative humidities. These three experimental series show that the protohaematites obtained in potassium halide powder mixtures are hydrophilic, whereas those obtained in discs are hydrophobic. These differences in the surface properties may indicate that crystal perfection during the recrystallization stage begins in the Fe₂O₃/K-salt interface.