Thermodynamic properties, low-temperature heat-capacity anomalies, and single-crystal X-ray refinement of hydronium jarosite, (H3 O)Fe3(SO4)2(OH)6

J. Majzlan, R. Stevens, J. Boerio-Goates, B. F. Woodfield, A. Navrotsky, P. C. Burns, M. K. Crawford, T. G. Amos

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64 Scopus citations

Abstract

Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)-SO4 solutions. Single-crystal XRD refinement with R1 = 0.0232 for the unique observed reflections ( Fo > 4σF and wR2 = 0.0451 for all data gave a = 7.3559(8) Å, c = 17.019(3) Å, Vo = 160.11(4) cm3, and fractional positions for all atoms except the H in the H3O groups. The chemical composition of this sample is described by the formula (H3O) 0.91Fe2.91(SO4)2[(OH) 5.64(H2O)0.18]. The enthalpy of formation (ΔHof) is -3694.5 ± 4.6 kJ mol-1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, γ-FeOOH, MgO, H2O, and α-MgSO4. The entropy at standard temperature and pressure (So is 438.9 ± 0.7 J mol-1 K-1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (Cp) data between 273 and 400 K were fitted to a Maier-Kelley polynomial Cp(T in K) = 280.6 + 0.6149T - 3199700T-2. The Gibbs free energy of formation is - 3162.2 ± 4.6 kJ mol-1. Speciation and activity calculations for Fe(III) - SO4 solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the Cp data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in Cp, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H3O)Fe3(SO4)2(OH) 6 was estimated ΔGof = -3226.4 ± 4.6 kJ mol-1, ΔHof = -3770.2 ± 4.6 kJ mol-1, So = 448.2 ± 0.7 J mol-1 K-1, Cp(T in K) = 287.2 + 0.6281T-3286000T-2 (between 273 and 400 K).

Original languageEnglish (US)
Pages (from-to)518-531
Number of pages14
JournalPhysics and Chemistry of Minerals
Volume31
Issue number8
DOIs
StatePublished - Oct 2004
Externally publishedYes

Keywords

  • Crystal structure
  • Entropy
  • Formation enthalpy
  • Hydronium jarosite
  • Spin glass

ASJC Scopus subject areas

  • Materials Science(all)
  • Geochemistry and Petrology

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