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The Transformation of Lepidocrocite During Heating: A Magnetic and Spectroscopic Study

Published online by Cambridge University Press:  28 February 2024

A. U. Gehring  and
A. M. Hofmeister
A. U. Gehring*
Affiliation:
Department of Soil Science, University of California, Berkeley, California 94720
A. M. Hofmeister
Affiliation:
Department of Geology, University of California, Davis, California 95616
*
1Present address: Swiss Federal Institute for Forest, Snow and Landscape Research, 8903, Birmensdorf, Switzerland
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Abstract

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Infrared (IR) spectroscopy, in combination with magnetic methods, was used to study the thermally induced transformation of synthetic lepidocrocite (γ-FeOOH) to maghemite (γ-Fe2O3). Magnetic analyses showed that the thermal conversion began at about 175°C with the formation of superparamagnetic maghemite clusters. The overall structural transformation to ferrimagnetic γ-Fe2O3 occurred at 200°C and was complete around 300°C. At higher temperatures, the maghemite converted into hematite (α-Fe2O3). Observation of the transformation from γ-FeOOH to γ-Fe2O3 using variable-temperature IR spectroscopy indicated that dehydroxilation on a molecular level was initiated between 145°C and 155°C. The lag time between the onset of the breaking of OH bonds and the release of H2O from lepidocrocite around 175°C can be explained by diffusive processes. Overall dehydroxilation and the subsequent breakdown of the lepidocrocite structure was complete below 219°C. The comparison of the magnetic and IR data provides evidence that the dehydroxilation may precede the structural conversion to maghemite.

Keywords

DehydroxilationIR spectraLepidocrociteMaghemitePhase transformation
Type
Research Article
Information
Clays and Clay Minerals , Volume 42 , Issue 4 , August 1994 , pp. 409 - 415
Copyright
Copyright © 1994, Clay Minerals Society

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