Evidence for the Fe(II)-Fe(III) Green Rust "Fougérite" mineral occurrence in a hydromorphic soil and its transformation with depth
Résumé
Mössbauer spectroscopy is used to characterize a Green Rust as a natural mineral for the first time. Samples are taken from a hydromorphic soil under the forest at Fougères (Brittany, France) during spring season. Spectra are compared with those of synthetic Green Rusts (GRs), i.e. hydroxy-chloride, -sulphate, -carbonate and display one ferric and one or two ferrous sites. Fe(II)/Fe(III) abundances ratio cannot be attributed definitely to one of those stoichiometric compounds. Therefore, the counter-anion species cannot be specified even though hyperfine parameters perfectly match values of synthetic samples. The Fe(II)/Fe(III) ratio obtained from the Mössbauer spectra decreases with sampling depth. The "fougerite" originates likely from the reduction of deeper Fe(III)-mineral species by anoxic waterlogging.