The Synthesis of Green Rust II(FeIII1–FeII2) and Its Spontaneous Transformation into Fe3O4

Conditions for the synthesis of a pure green rust II with a definite chemical composition and its transformation reaction to Fe3O4 under a nitrogen atmosphere were studied. A pure green rust II with the composition of (Fe3+)1(Fe2+)2(SO42−)(OH−)5−2n(O2−)n was obtained by the air oxidation of an aqueous solution containing (0.05 mol dm−3) FeII and (0.3 mol dm−3) SO42− ions at pH 6.8 and 25 °C. This iron composition differs from those estimated previously; therefore, the green rust II synthesized here is designated as green rust II(FeIII1–FeII2). This green rust II is spontaneously transformed into Fe3O4 under a nitrogen atmosphere at 50–75 °C. It has been accepted that green rust II is transformed to Fe3O4 by oxidation. The above evidence suggests that the green rust II can be transformed into Fe3O4 not only by oxidation but also spontaneously without oxidants.