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

This study explores the thermodynamic properties and structural characterization of hydronium jarosite, a mineral synthesized through hydrothermal treatment of Fe(III)–SO4 solutions. The objective was to determine the formation enthalpy, entropy, and heat capacity, as well as to refine its crystal structure via single-crystal X-ray diffraction (XRD). Utilizing statistical techniques such as the Maier-Kelley polynomial fitting, the heat capacity was analyzed across a temperature range of 273 to 400 K. The research yielded precise estimates of the enthalpy of formation at 3694.5 ± 4.6 kJ/mol and entropy at 438.9 ± 0.7 J/mol·K, corroborating earlier findings from solubility experiments. Notably, the study identified low-temperature heat capacity anomalies attributed to a spin-glass freezing transition and potential spin cluster tunneling behavior. Further calculations on an ideal composition end member generated a Gibbs free energy of formation of -3226.4 ± 4.6 kJ/mol. The results collectively enhance the understanding of hydronium jarosite's thermodynamics, contributing valuable data for geological and mineralogical applications.