Formation of green rust via mineralogical transformation of ferric oxides (ferrihydrite, goethite and hematite)

This laboratory study describes the abiotic mineralogical transformation of ferrihydrite (F), goethite (G) and hematite (H) into mixed FeII–FeIII green rust (GR). Stoichiometric quantities of ferric oxides were reacted with hydroxylated FeII species under static batch conditions at a final pH of ~ 6.3 for 2 h. Mössbauer spectroscopy and transmission electron microscopy (TEM) was used to characterize initial and resulting transformation products. Results indicate that the order of reactivity to transform into GR is F > G > H. Complete transformation of F into GR was achieved while about half of G or H was transformed although G or H are known for their higher thermodynamic stability. As expected, the concentration of soluble iron at the end of the experiment followed the order H > G > F which is in agreement with the predictions of the FeII–FeIII mass balance diagram. The formation kinetics of GR is much higher than that observed for magnetite (Fe3O4) under similar experimental conditions. Therefore in the environment, when hydroxylated FeII species interact with ferric oxides, fougerite, the mineral counter part of GR, could be preferentially formed.