Formation of siderite and vivianite and the pore-water composition of a Recent bog sediment in Denmark

In a bog deposit containing siderite and vivianite, both minerals consist of a solid solution with Mn2+ and Ca2+. It is found that Mn is preferentially concentrated in the carbonate and also that the compositional variation within single samples is large. High Fe2+ concentrations are found in the pore water, while H2S is absent in agreement with thermodynamic predictions. Calculated saturation states for pure end-member minerals show near equilibrium with respect to rhodochrosite while with siderite up to a magnitude of supersaturation is found. Also strong supersaturation is found with respect to vivianite while with Mn3 (PO4)2 strong subsaturation prevails. Supersaturation with both siderite and vivianite cannot be explained by solid solution alone, and it is shown that their saturation states are largely controlled by pH variations. Processes which may cause the precipitation of siderite within and not outside the bog are reduction of ferric oxyhydroxides and dissolution of CaCO3 from the solid carbonate. The present data do not allow a quantitative expression for the importance of these processes. In a particular case, however, it could be shown that equilibrium with calcite controls the saturation state for siderite.