Pure Appl. Chem., 2008, Vol. 80, No. 6, pp. 1317-1323
Synthesis and stability of wroewolfeite, Cu4SO4(OH)6·2H2O
Titration of aqueous copper(II) sulfate solutions with aqueous NaOH at temperatures ranging from 0 to 25 °C results in a complex Ostwald step rule cascade of basic copper sulfate phases. At 25 °C, the thermodynamically stable phase is brochantite [Cu4SO4(OH)6], but posnjakite [Cu4SO4(OH)6·H2O] is formed first. At lower temperatures, but above 0 °C, wroewolfeite [Cu4SO4(OH)6·2H2O] forms first. If left in contact with the reaction solution, wroewolfeite is converted to posnjakite and brochantite in turn. However, at 0 °C, synthetic wroewolfeite is stable for periods longer than a week, even in contact with the reaction solution, and a stability constant could be determined for its formation. For the reaction below, lg K = -16.3(1) at 0 °C and I = 0, as determined by solution methods.
0.25Cu4SO4(OH)6·2H2O(s,wroewolfeite) = Cu2+(aq) + 0.25SO42-(aq) + 1.5OH-(aq) + 0.5H2O(l)
Stability relations between minerals of stoichiometry Cu4SO4(OH)6·2H2O (n = 0, 1, 2) are discussed. High concentrations of Mg2+ ions (1 M) prevent the isolation of wroewolfeite at any temperature down to 0 °C.