Composition, stabilization, and light absorption of Fe(II)-Fe(III) hydroxycarbonate (green

ABSTRACT: The chemical composition of the pyroaurite-type compound Fe(II)Fe(III) hydroxy-carbonate ('green rust') synthesised from freshly precipitated ferrihydrite and Fe(II) chloride solution at pH 7.0 ('induced hydrolysis') was determined. The compound was nearly stoichiometric, with the formula FeInFe[l(OH)12CO3, the Fe(II):Fe(III) ratio being indepen-dent of the Fe(II):Fe(III) ratio in the initial reaction mixture. It shows all the XRD peaks reported for this compound. Visible-near IR shows abroad peak at 650 nm which is ascribable to intervalence harge transfer and therefore the absorbance maximum decreases with increasing degree of oxidation. Wetting the material with compounds containing hydroxyl groups (such as glycerol or glucose) retards the oxidation of the otherwise very oxidation-sensitive compound. Similar polar organic ompounds may stabilize the Fe(II)Fe(III) hydroxy-carbonate in wet soils. The importance ofthe green rust phase as an intermediate in the formation and transformation reactions of oxides and oxyhydroxides of iron in natural environments should not be ignored. Green rust compounds are Fe(II)Fe(III) hydroxy-compounds containing a certain amount of a non-hydroxyl anion which is reported to be sulphate, bromide, fluoride (Bernal et al., 1959), chloride (Feitknecht & Keller, 1950), iodide (Vins et al., 1987). nitrate (Gancedo et al., 1983