A Mössbauer and chemical study of the formation of vivianite in sediments of Lago Maggiore (Italy)

The interstitial waters and the solid phase of a core collected in Lago Maggiore (Italy) were analysed principally for Fe and P. Thermodynamic equilibrium calculations from the data of the pore fluids show that the interstitial waters are oversaturated with respect to vivianite in the upper part of the core. "Vivianite" was detected by optical microscopy as brown-orange concretions at depth below 3 cm and as blue concretions below 5.5 cm. At the 7–8 cm level these concretions were sufficiently large to be separated from the sediment and SEM-EDAX analysis showed the blue and the brown-orange concretions to have similar compositions. X-ray diffraction identified the blue concretions as vivianite, which Mössbauer spectroscopy showed to be partially oxidized. The brown-orange concretions were identified as fully oxidized vivianite. Total P and total Fe profiles in the core together with sedimentation data help to explain the presence of these large concretions at a depth of 7–8 cm.