Green rust-like compounds (GRs) were discovered as natural minerals in various hydromorphic soils, where anoxic conditions allow their stability. They may control some redox processes in aquifers and participate to the transformation of various pollutants. Since Mg(II) cations are present in the fields where GRs were discovered, a partial substitution of Mg(II) to Fe(II) leading to intermediate compounds between GRs and usual Mg(II)–Fe(III) hydroxysalts is suspected. Mg(II)–Fe(II)–Fe(II) hydroxycarbonates can be obtained as intermediate oxidation products of (Mg, Fe)(OH)2 in carbonate-containing aqueous media obeying to [FeII 4(1−x)MgII 4x FeIII 2(OH)12]2+ ⋅[CO3 2− ⋅ nH2O]−2. TMS spectra at 12 K are similar to those of GRs, i.e., two quadrupole doublets, one due to Fe(II) with a large isomer shift δ=1.29 mm s−1 (with respect to α-iron at room temperature) and quadrupole splitting ΔE Q=2.76 mm s−1, the other one due to Fe(III) with smaller hyperfine parameters δ=0.49 mm s−1 and ΔE Q=0.44 mm s−1. Fe(II) ions oxidise rapidly into Fe(III) with dissolved O2. The reactivity is similar to that of Fe(II)–Fe(III) hydroxysalts GR, and thus the potential of Mg(II)–Fe(II)–Fe(III) compounds for reducing pollutants.