Study of the formation processes of iron oxy-hydroxides; hypotheses of topotactic transformations

Goethite alpha-FeO(OH) is the most stable iron oxy-hydroxide. It can be formed directly from trivalent iron solutions or from the 'white rust' (WR): Fe(OH)(2). by oxidation of divalent iron. Nevertheless, another phase gamma-FeO(OH), named lepidocrocite, can be obtained, at room temperature, in particular conditions of formation: fast oxidation of divalent iron in a precursor phase, named 'green rust' (GR), containing also trivalent iron and chloride ions. The formation of lepidocrocite from GR cannot be explained by a reconstructive transformation, which involves dissolution and re-crystallization. A structural study and a comparison of crystal structures of these different phases have been carried out. On the basis of that study, we propose three possible hypotheses of topotactic reactions: GR --> lepidocrocite; WR --> goethite, and lepidocrocite --> goethite These transformations bring some explanations about the formation of iron oxy-hydroxides, which agree well with experimental results. (C) 2003 Academie des sciences. Publie par Editions scientifiques et medicales Elsevier SAS. Tous droits reserves.