In situ Mössbauer spectroscopy : Evidence for green rust (fougerite) in a gleysol and its mineralogical transformations with time and depth

A miniaturized Mössbauer spectrometer, adapted to the Earth's conditions from the instrument developed for Mars space missions, has been used for the first time to study in situ variations with depth and transformations with time of iron minerals in a gleysol. The instrument is set into a PVC tube and can be moved up and down precisely (± 1 mm) at the desired depth. Mössbauer spectra were obtained from 15 to 106 cm depth and repeated exactly at the same point at different times to follow mineralogical transformations with time. X-ray diffraction (XRD) and selective extraction techniques were performed on soil samples. The piezometric level of the water table was measured and the composition of the soil solution was monitored in situ and continuously, with a multiparametric and automatic probe. All the Mössbauer spectra obtained are characteristic of Fe(II)-Fe(III) green rust-fougerite, a natural mineral of the meixnerite group, that is, whose structural formula is: [FeII1- x Mgy FeIIIx (OH)2+2y]x+[xA, mH2O]x-, where x is the ratio Fe3+/Fetot. and A the intercalated anion. The name of fougerite has been formally approved by the Commission on New Minerals and Mineral Names of IMA (number 2003-057), on January 29, 2004. No other iron phases have been found by this way or by XRD. About 90% of total iron is extractible by dithionite-citrate-bicarbonate, and 60% by citratebicarbonate. In the horizons showing oximorphic properties that are in the upper part of the studied soil profile, x ratio in fougerite, deduced from Mössbauer spectra, is approximately 2/3. In the deepest horizons that show reductomorphic properties, x ratio is only 1/3. Fast mineralogical. transformations were observed at well-defined points in soil, as evidenced by x ratio variations observed when Mössbauer spectra were acquired at different times at the same depth. Variations of the level of the water table and of pe and pH of the soil solution were simultaneously observed and could explain these mineralogical transformations. A ternary solid solution model previously proposed for OH-fougerite has been extended to chloride, sulphate, and carbonate green rusts to estimate the Gibbs free energies of formation of fougerite, providing for possible anions other than OH- in the interlayer and for Mg substitution. Soil solutions appear as largely oversaturated with respect to OH-fougerite, either oversaturated or undersaturated to "carbonate-fougerite" and "sulphate-fougerite", and largely undersaturated with respect to "chloro-fougerite". Fougerite forms most likely from oversaturated solutions by coprecipitation of Fe3+ with Fe2+ and Mg2+. Oxidation and reduction are driven by pH and pe variations, with both long timescale variations and short duration events. Exactly as synthetic green rusts are very reactive compounds in the laboratory, fougerite is thus a very reactive mineral and readily forms, dissolves, or evolves in soils. (Résumé d'auteur