Mixtures of Fe(OH) 2 and Fe 2 (OH) 2 CO 3 were obtained by precipitation from FeCl 2 , NaOH and Na 2 CO 3 solutions. These two-phase systems did not change with time and equilibrium conditions were supposed to be established after 6months. The standard Gibbs free energy of formation of chukanovite could then be estimated at −1171.5±3.0kJmol −1 . In contrast, mixtures of FeCO 3 and Fe 2 (OH) 2 CO 3 could not been obtained by precipitation but siderite formed in some cases after 25months of ageing. Pourbaix diagrams of iron including chukanovite were drawn showing that the Fe(II) hydroxycarbonate was metastable, at 25°C, with respect to siderite.