Majzlan, J; Amoako, FY; Kindlova, H; Drahota, P
Zykaite, nominally Fe-4(AsO4)(3)(SO4)(OH)center dot 15H(2)O, 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 Fe-1.23(AsO4)(0.93)(PO4)(0.07)(SO4)(0.31)(OH)(0.07) center dot 5.89H(2)O and typical poor crystallinity. The solubility product, derived from solubility experiments, is log K-sp = -25.77 for the reaction Fe-1.23(AsO4)(0.93)(PO4)(0.07) (SO4)(0.31)(OH)(0.07)center dot 5.89H(2)O + 0.07 H+ = 1.23Fe(3+) + 0.93AsO(4)(3+) + 0.07PO(4)(3+) + 0.31SO(4)(2+) + 5.96H(2)O. Enthalpy of formation, determined by acid-solution calorimetry, is -2952.2 +/- 2.9 kJ mol(-1). Gibbs free energy of formation, calculated from the log K-sp value, is -2485.1 kJ mol(-1). The entropies of zykaite, 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 zykaite gradually collapses and turns amorphous at similar to 90 degrees C. No crystalline phase formed upon heating to 200 degrees C. Zykaite 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). Zykaite and kankite (FeAsO4 center dot 3.5H(2)O) 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 zykaite to other ferric arsenates and sulfoarsenates (scorodite, parascorodite, kan. kite, bukovskyite, hydrous ferric arsenates) is thermodynamically driven. Comparing and combining our thermodynamic data, modeling, variable-temperature X-ray diffraction, and field observations, it seems that zykaite transforms readily to other phases in the system Fe2O3-As2O5-SO3-H2O via dissolution and precipitation; a solid-state transformation is unlikely. (C) 2015 Elsevier Ltd. All rights reserved.