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

A. U. Gehring1 and A. M. Hofmeister2
1 Department of Soil Science, University of California, Berkeley, California 94720
2 Department of Geology, University of California, Davis, California 95616
1 Present address: Swiss Federal Institute for Forest, Snow and Landscape Research, 8903 Birmensdorf, Switzerland.

Abstract: 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 super-paramagnetic 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.

Key Words: Dehydroxilation • IR spectra • Lepidocrocite • Maghemite • Phase transformation

Clays and Clay Minerals; August 1994 v. 42; no. 4; p. 409-415; DOI: 10.1346/CCMN.1994.0420405
© 1994, The Clay Minerals Society
Clay Minerals Society (www.clays.org)