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Mössbauer and XAS study of a green rust mineral; the partial substitution of Fe²⁺ by Mg²⁺

¹ Laboratoire de Chimie Physique et Microbiologie pour l’Environnement, UMR 7564 CNRS-Université Henri Poincaré, F-54600 Villers-lès-Nancy, France
² INRA-Géochimie des Sols et des Eaux, CEREGE, BP80, F-13545 Aix-en-Provence Cedex 4, France

Layered double hydroxysalt green rusts, GRs, are very reactive compounds and their general formula, [Fe_(1–x)²⁺ Fe_x³⁺ (OH)₂]^x+ [x/n A^n–·m H₂O]^x–, where x is the ratio Fe³⁺/Fe_tot, reflects the structure in which brucite-like layers alternate with interlayers of anions A^n– and water molecules. A GR mineral was extracted from hydromorphic soils in Fougères (France) and studied by X-ray absorption spectroscopy (XAS) and transmission Mössbauer spectroscopy (TMS). The XAS spectrum at the Fe K absorption edge of this mineral proved to be very similar to that of synthetic GRs. However, the radial distribution function obtained for the GR mineral proved to be intermediate between those of GR(CO₃^2–) and pyroaurite, that is between the Fe²⁺-Fe³⁺ and Mg²⁺-Fe³⁺ hydroxycarbonates. Consequently, a partial substitution of Fe²⁺ by Mg²⁺ occurs, leading to the general formula of [Fe_(1–x)²⁺Mg_y²⁺Fe_x³⁺(OH)_(2+2y)]^x+ [x/n A^n–·m H₂O]^x– where A^n– is the interlayer anion. Unfortunately, the XAS spectra of various GR proved to be independent of the interlayer anion, and the nature of the anions present in the mineral GR could not be determined. The Mössbauer spectrum of the mineral, measured at 77 K, is composed of four quadrupole doublets: D₁ and D₂ due to Fe²⁺ [ 1.26 mm/s and E_Q 2.5 and 2.9 mm/s, respectively] and D₃ and D₄ due to Fe³⁺ [ 0.46 mm/s and E_Q 0.5 and 1.0 mm/s, respectively]. Finally, synthetic Mg²⁺-Fe²⁺-Fe³⁺ hydroxycarbonates could be prepared by coprecipitation from Mg and Fe salts and lead to Mössbauer spectra similar to that of the mineral. In particular, the partial substitution of Fe²⁺ by Mg²⁺ proved to be consistent with the existence of the unusual doublet D₄.

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