Titre du document / Document title

THERMODYNAMIC PROPERTIES OF FEROXYHYTE (δ'-FeOOH)

Auteur(s) / Author(s)

MAJZLAN Juraj ⁽¹⁾ ; KOCH Christian Bender ⁽²⁾ ; NAVROTSKY Alexandra ⁽³⁾ ;

Affiliation(s) du ou des auteurs / Author(s) Affiliation(s)

⁽¹⁾ Institute of Mineralogy and Geochemistry, Albertstraße 23b, Albert-Ludwig University, Freiburg, 79104, ALLEMAGNE
⁽²⁾ Department of Natural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871 Frederiksberg, DANEMARK
⁽³⁾ Thermochemistry Facility, One Shields Avenue, University of California at Davis, 95616 Davis, CA, ETATS-UNIS

Résumé / Abstract

-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 is always fine-grained, although samples with larger particle sizes and better crystallinity (labeled as 5-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±1.3 kJ mol^-1 for the poorly crystalline sample (surface area 88 m /g), and -550.6±1.4, -550.9±1.3, and -552.6±1.2kJ mole^-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 S°₂₉₈ 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 (ΔG°_f) 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 ΔG°_f 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.

Revue / Journal Title

Clays and clay minerals   ISSN 0009-8604   CODEN CLCMAB 

Source / Source

2008, vol. 56, n^o5, pp. 526-530 [5 page(s) (article)] (1/2 p.)

Langue / Language

Anglais

Editeur / Publisher

Clay Minerals Society, Boulder, CO, ETATS-UNIS  (1968) (Revue)

Mots-clés anglais / English Keywords

iron oxides ; hydroxides ; stability ; maghemite ; lepidocrocite ; thermodynamics ; physical properties ; hematite ; free energy ; magnetic properties ; entropy ; enthalpy ; natural materials ; crystallinity ; particles ; grains ; fine-grained materials ; oxides ; thermodynamic properties ;

Mots-clés français / French Keywords

Oxyde fer ; Hydroxyde ; Stabilité ; Maghemite ; Lepidocrocite ; Thermodynamique ; Propriété physique ; Hematite ; Energie libre ; Propriété magnétique ; Entropie ; Enthalpie ; Matériau naturel ; Cristallinité ; Particule ; Grain ; Fraction fine ; Oxyde ; Propriété thermodynamique ;

Mots-clés espagnols / Spanish Keywords

Óxido de hierro ; Hidróxido ; Estabilidad ; Maghemita ; Lepidocrocita ; Termodinámica ; Propiedad física ; Hematites ; Energía libre ; Propiedad magnética ; Entropía ; Entalpía ; Cristalinidad ; Grano ; Fracción fina ; Óxido ;

Mots-clés d'auteur / Author Keywords

Enthalpy of Formation ; Feroxyhyte ; Thermodynamic Stability ;

Localisation / Location

INIST-CNRS, Cote INIST : 14383, 35400018513243.0060