Layered double hydroxysalt green rusts, GRs, are very reactive compounds and their general formula, [Fe(1−x)2+ Fex3+ (OH)2]x+ [x/n An−·m H2O]x−, where x is the ratio Fe3+/Fetot, reflects the structure in which brucite-like layers alternate with interlayers of anions An− 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(CO32−) 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+Mgy2+Fex3+(OH)(2+2y)]x+ [x/n An−·m H2O]x− where An− 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+ [δ ≅ 1.26 mm/s and ΔEQ ≅ 2.5 and 2.9 mm/s, respectively] and D3 and D4 due to Fe3+ [δ ≅ 0.46 mm/s and ΔEQ ≅ 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.