MOSSBAUER STUDY OF SYNTHETIC OXIDIZED VIVIANITE AT ROOM-TEMPERATURE

Mossbauer spectroscopy has been used to study synthetic vivianites which are oxidized at room temperature in air. Six doublets, three ferrous and three ferric, have been used to fit the spectra recorded at 295 K. They have been attributed to the cations occupying the two different crystallographic sites. These sites were either isolated (I) or in pairs (II). In the case of the paired sites, two types of ferric cations and two types of ferrous cations can be distinguished, depending upon 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 oxidation process and that the rates of oxidation of the cations occupying two sites were comparable at a high oxidation level. We have also observed that the concentration of Fe3+ tends to a stabilized value of approximately 50% after 375 days, which also corresponds to the limit of stability of the vivianite structure.