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.