Preparation and Thermodynamic Equilibria of Green Rusts in Aqueous Solutions and their Identification as Minerals in Hydromorphic Soils

Abstract

Many natural minerals are made by stacking positively and negatively charged layers. Among them, the pyroaurite-sjogrenite group (Allmann et Lohse, 1966; Allmann, 1968) corresponds to the stacking of brucite-like layers carrying a positive charge and layers constituted of anions and water molecules. In this paper, we deal with the Fe(II)-Fe(III) compounds generally known by the generic name of Green Rusts, GRs, since the original classification by Bernai et al. (1959) and the major anions which are found in natural waters, i.e. Cl −, SO42− and CO32− are concerned. From X-ray diffraction (XRD), two types of GRs are distinguished, Green Rust one (GR1) incorporating “planar anions”, e.g. Cl− and CO32su−, and Green Rust two (GR2) incorporating “three dimensional anions”. The preparation of these compounds in the laboratory by controlled oxidation of a ferrous hydroxide precipitate in anion - containing aqueous solutions allows us, by monitoring the E h and pH versus time curves to elaborate them at stoichiometry with the appropriate [Fe++] / [OH−] concentration ratio. The X-ray diffraction characterization of these “Green Rusts”, and the corresponding Mossbauer spectra allow us to determine structural information and chemical formulae