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Thermodynamic Properties of Feroxyhyte (δ′-FeOOH)

Published online by Cambridge University Press:  01 January 2024

Juraj Majzlan ,
Christian Bender Koch  and
Alexandra Navrotsky
Juraj Majzlan*
Affiliation:
Institute of Mineralogy and Geochemistry, Albertstraße 23b, Albert-Ludwig University, Freiburg, D-79104 Germany
Christian Bender Koch
Affiliation:
Department of Natural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
Alexandra Navrotsky
Affiliation:
Thermochemistry Facility, One Shields Avenue, University of California at Davis, 95616 Davis, CA, USA
*
* E-mail address of corresponding author: [email protected]
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Abstract

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Feroxyhyte (δ′-FeOOH) is a relatively uncommon Fe oxide mineral and one of the few phases in the system Fe2O3-H2O for which thermodynamic properties are not known. In natural occurrences, it is always fine-grained, although samples with larger particle sizes and better crystallinity (labeled as δ-FeOOH) can be prepared in the laboratory. This contribution presents a thermochemical study on a series of feroxyhyte samples. One is fine-grained and poorly crystalline, similar to natural materials, while the other three are of better crystallinity. The enthalpy of formation of feroxyhyte at 298.15 K is −547.4±1.3kJ mol−1 for the poorly crystalline sample (surface area 88 m2/g), and −550.6±1.4, −550.9±1.3, and −552.6±1.2 kJ mol−1 for the samples with better crystallinity. The entropy of feroxyhyte can be estimated only crudely, because it is influenced to a great extent by its magnetic properties, particle size, and structural disorder. The S298o$S_{298}^{\rm{o}}$ of feroxyhyte is estimated here to be 65±5 J K−1 mol−1. The Gibbs free energy of the reaction feroxyhyte → hematite + liquid water is −7.4 to −12.6 kJ mol−1 at 298.15 K. The Gibbs free energy of formation (ΔGfo${\rm{\Delta }}G_{\rm{f}}^{\rm{o}}$) of the fine-grained, poorly crystalline feroxyhyte is −478.1±2.0 kJ mol−1 at 298.15 K. Since this sample is closest in its physical properties to natural feroxyhyte, this ΔGfo${\rm{\Delta }}G_{\rm{f}}^{\rm{o}}$ value should be used in thermodynamic modeling related to processes involving naturally occurring feroxyhyte. In terms of Gibbs free energy and enthalpy, feroxyhyte is very similar to lepidocrocite and maghemite, and, like these two phases, has no thermodynamic stability field in the system Fe2O3-H2O, except possibly at the nanoscale.

Keywords

Enthalpy of FormationFeroxyhyteThermodynamic Stability
Type
Article
Information
Clays and Clay Minerals , Volume 56 , Issue 5 , October 2008 , pp. 526 - 530
Copyright
Copyright © The Clay Minerals Society 2009

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