MOSSBAUER STUDY OF SYNTHETIC OXIDIZED VIVIANITE AT ROOM-TEMPERATURE

Mossbauer spectroscopy has been used to study synthetic vivianites whi ch are oxidized at room temperature in air. Six doublets, three ferrou s and three ferric, have been used to fit the spectra recorded at 295 K. They have been attributed to the cations occupying the two differen t crystallographic sites. These sites were either isolated (I) or in p airs (II). In the case of the paired sites, two types of ferric cation s and two types of ferrous cations can be distinguished, depending upo n the degree of oxidation of the cation occupying the closest isolated site. Our experimental data showed that the ferrous cations occupying sites I were preferentially oxidized at the beginning of the oxidatio n process and that the rates of oxidation of the cations occupying two sites were comparable at a high oxidation level. We have also observe d that the concentration of Fe3+ tends to a stabilized value of approx imately 50% after 375 days, which also corresponds to the limit of sta bility of the vivianite structure.