Abstract
In this work, we investigated the thermodynamic properties of synthetic schafarzikite (FeSb2O4) and tripuhyite (FeSbO4). Low-temperature heat capacity ( C p) was determined by relaxation calorimetry.
From these data, third-law entropy was calculated as 110.7 +/- 1.3 J mol(-1)K(-1) for tripuhyite and 187.1 +/- 2.2 J mol(-1) K-1 for schafarzikite. Using previously published Delta(f)G degrees values for both phases, we calculated their Delta H-f degrees as -947.8 +/- 2.2 for tripuhyite and -1061.2 +/- 4.4 for schafarzikite.
The accuracy of the data sets was tested by entropy estimates and calculation of Delta H-f degrees 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.