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

Abstract

AbstractIn this work, we investigated the thermodynamic properties of synthetic schafarzikite (FeSb2O$$_4$$ 4 ) and tripuhyite (FeSbO$$_4$$ 4 ). Low-temperature heat capacity ($$C_p$$ C p ) was determined by relaxation calorimetry. From these data, third-law entropy was calculated as $$110.7\pm 1.3$$ 110.7 ± 1.3 J mol$$^{-1}$$ - 1 K$$^{-1}$$ - 1 for tripuhyite and $$187.1\pm 2.2$$ 187.1 ± 2.2 J mol$$^{-1}$$ - 1 K$$^{-1}$$ - 1 for schafarzikite. Using previously published $$\Delta _fG^o$$ Δ f G o values for both phases, we calculated their $$\Delta _fH^o$$ Δ f H o as $$-947.8\pm 2.2$$ - 947.8 ± 2.2 for tripuhyite and $$-1061.2\pm 4.4$$ - 1061.2 ± 4.4 for schafarzikite. The accuracy of the data sets was tested by entropy estimates and calculation of $$\Delta _fH^o$$ Δ f H o from estimated lattice energies (via Kapustinskii equation). Measurements of $$C_p$$ C p above $$T = 300$$ T = 300 K were augmented by extrapolation to $$T = 700$$ T = 700 K with the frequencies of acoustic and optic modes, using the Kieffer $$C_p$$ C p model. A set of equilibrium constants ($$\log K$$ 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$$\epsilon$$ ϵ conditions at $$T = 298.15$$ 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.