Thermodynamics of schafarzikite (FeSb2O\(_4\)) and tripuhyite (FeSbO\(_4\))

In this work, we investigated the thermodynamic properties of synthetic schafarzikite (FeSb₂O\(_4\)) and tripuhyite (FeSbO\(_4\)). Low-temperature heat capacity (\(C_p\)) was determined by relaxation calorimetry. From these data, third-law entropy was calculated as \(110.7\pm 1.3\) J mol\(^{-1}\)K\(^{-1}\) for tripuhyite and \(187.1\pm 2.2\) J mol\(^{-1}\) K\(^{-1}\) for schafarzikite. Using previously published \(\Delta _fG^o\) values for both phases, we calculated their \(\Delta _fH^o\) as \(-947.8\pm 2.2\) for tripuhyite and \(-1061.2\pm 4.4\) for schafarzikite. The accuracy of the data sets was tested by entropy estimates and calculation of \(\Delta _fH^o\) from estimated lattice energies (via Kapustinskii equation). Measurements of \(C_p\) above \(T = 300\) K were augmented by extrapolation to \(T = 700\) K with the frequencies of acoustic and optic modes, using the Kieffer \(C_p\) model. A set of equilibrium constants (\(\log K\)) for tripuhyite, schafarzikite, and several related phases was calculated and presented in a format that can be employed in commonly used geochemical codes. Calculations suggest that tripuhyite has a stability field that extends over a wide range of pH-p\(\epsilon\) conditions at \(T = 298.15\) K. Schafarzikite and hydrothermal oxides of antimony (valentinite, kermesite, and senarmontite) can form by oxidative dissolution and remobilization of pre-existing stibnite ores.