Formation and properties of Fe(II)Fe(III) hydroxy-carbonate and its possible significance in soil formation

Abstract

AbstractSmall amounts of Fe(III) induce precipitation of Fe(II) from carbonate solutions around neutral pH values. The reaction involves the formation of Fe(II)Fe(III) hydroxycarbonate, a green, layer-structured compound of the pyroaurite group of minerals. Variations in the concentration of Fe(II) in the solution phase, and in the form and amounts of the total Fe(III) in the system, may cause other phases such as magnetite and siderite to form also. Freeze-dried Fe(II)Fe(III) hydroxy-carbonate undergoes topotactic alteration to a magnetic phase, presumably maghemite, when stored under vacuum. The compound is readily oxidized in air in either the moist or dry state, rapidly becoming yellow or yellow-brown in colour. Oxidation leads to the formation of either goethite, lepidocrocite, ferrihydrite or mixtures of these phases, depending on the mode of oxidation and the impurities present. The similarity between the rapid colour changes and the oxidation products of the blue-green precipitates in gleyed soil horizons and this Fe(II)Fe(III) hydroxy-carbonate leads to the speculation that this and related compounds may be the meta-stable precipitates in such soils, rather than a previously postulated hydro-magnetite phase. Eh and pH conditions for the formation of Fe(II)Fe(III) hydroxy-carbonate are given, and are in the range encountered in the soil environment.