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 [Cu 4SO4(OH)6·H2O] 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) + 0.25SO 4 2-(aq) + 1.5OH-(aq) + 0.5H2O(l) Stability relations between minerals of stoichiometry Cu4SO 4(OH)6·nH2O (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.