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Composition, stabilization, and light absorption of Fe(II)Fe(III) hydroxy-carbonate (‘green rust’)

Published online by Cambridge University Press:  09 July 2018

H. C. B. Hansen*
Affiliation:
Royal Veterinary & Agricultural Highschool, Chemistry Department, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark

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 the Fe(II):Fe(III) ratio being independent 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 a broad peak at 650 nm which is ascribable to intervalence charge 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 compounds may stabilize the Fe(II)Fe(III) hydroxy-carbonate in wet soils. The importance of the 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.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1989

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