Thermodynamics of schafarzikite (FeSb2O_4) and tripuhyite (FeSbO_4)

Abstract

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 110.7pm 1.3 J mol^{-1}K^{-1} for tripuhyite and 187.1pm 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 -947.8pm 2.2 for tripuhyite and -1061.2pm 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-pepsilon 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.