Thermodynamic properties, low-temperature heat-capacity anomalies, and single-crystal X-ray refinement of hydronium jarosite, (H3O)Fe3(SO4)2(OH)6

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).