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Maghemite in soils and its origin

II. Maghemite syntheses at ambient temperature and pH 7

Published online by Cambridge University Press:  09 July 2018

R. M. Taylor  and
U. Schwertmann
R. M. Taylor
Affiliation:
Institut für Bodenkunde, Technische Universität München, Freising-Weihenstephan, West Germany
U. Schwertmann
Affiliation:
Institut für Bodenkunde, Technische Universität München, Freising-Weihenstephan, West Germany
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Abstract

The products of air oxidation of mixed Fe(II)-Fe(III) chloride solutions at pH 6 and 7, at 20 and 60°C and at normal pressure contain green rust, maghemite, lepidocrocite, goethite and a paracrystalline ferric hydroxide (ferrihydrite). Among these maghemite, a cubic ferromagnetic iron oxide (Fe2O3) found in many soils, is favoured by slow oxidation rate, high total Fe concentration, the presence of small amounts of Fe(III) in the original predominantly Fe(II) solution, higher temperature and at pH 7 rather than pH 6. The green rust is believed to be an essential precursor of maghemite. On slow oxidation it will form maghemite probably via magnetite. Fast oxidation prevents the cubic phase from being formed and lepidocrocite is the end product. At higher Fe(III) proportions ferrihydrite can be formed which under certain influences converts to goethite and/or hematite. The common iron oxides are seen to form from the same system from small variations in environment which is to be expected from their common associations in soils

Type
Research Article
Information
Clay Minerals , Volume 10 , Issue 4 , December 1974 , pp. 299 - 310
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1974

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