Thermodynamic properties of zýkaite, a ferric sulfoarsenate

Abstract

Zýkaite, nominally Fe4(AsO4)3(SO4)(OH)⋅15H2O, is a common ferric arsenate in damp, moist environments, especially in underground spaces where the humidity does not decline during the year and the temperature fluctuations are minimal. The natural sample used in this study had chemical composition Fe1.23(AsO4)0.93(PO4)0.07(SO4)0.31(OH)0.07⋅5.89H2O and typical poor crystallinity. The solubility product, derived from solubility experiments, is log Ksp=−25.77 for the reaction Fe1.23(AsO4)0.93(PO4)0.07(SO4)0.31(OH)0.07⋅5.89H2O+0.07 H+=1.23Fe3++0.93AsO43−+0.07PO43−+0.31SO42−+5.96H2O. Enthalpy of formation, determined by acid-solution calorimetry, is −2952.2±2.9kJmol−1. Gibbs free energy of formation, calculated from the log Ksp value, is −2485.1kJmol−1. The entropies of zýkaite, estimated for a solid and calculated from its enthalpy and Gibbs free energy of formation, are in a fair agreement and document the accuracy of the thermodynamic data. Variable-temperature powder X-ray diffraction showed that the structure of zýkaite gradually collapses and turns amorphous at ∼90°C. No crystalline phase formed upon heating to 200°C. Zýkaite is one of the least stable phases in the system Fe2O3–As2O5–SO3–H2O, restricted to environments with low pH and very high activities of As(V) and S(VI). Zýkaite and kaňkite (FeAsO4⋅3.5H2O) are often seen on the gently sloping rock faces or on the rock walls, whereas stalactites of hydrous ferric arsenate forms below them on the overhangs. Thermodynamic modeling with our data show that the transformation of zýkaite to other ferric arsenates and sulfoarsenates (scorodite, parascorodite, kaňkite, bukovskýite, hydrous ferric arsenates) is thermodynamically driven. Comparing and combining our thermodynamic data, modeling, variable-temperature X-ray diffraction, and field observations, it seems that zýkaite transforms readily to other phases in the system Fe2O3–As2O5–SO3–H2O via dissolution and precipitation; a solid-state transformation is unlikely.