Formation and properties of Fe(II)Fe(III) hydroxycarbonate and its possible significance insoil formation

ABSTRACT: Small amounts of Fe(IlI) induce precipitation f Fe(II) from carbonate solutions around neutral pH values. The reaction involves the formation of Fe(II)Fe(IlI) hydroxy-carbonate, a green, layer-structured compound of the pyroaurite group of minerals. Variations in the concentration f Fe(II) in the solution phase, and in the form and amounts of the total Fe(III) in the system, may cause other phases uch as magnetite and siderite to form also. Freeze-dried Fe(II)Fe(III) hydroxy-carbonate undergoes topotactic alteration to a magnetic phase,!~resum-ably maghemite, when stored under vacuum. The compound is readily oxidized in air in either the moist or dry state, rapidly becoming yellow or yeUow-brown i 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 his 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