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Pure Appl. Chem., 2008, Vol. 80, No. 6, pp. 1317-1323

Synthesis and stability of wroewolfeite, Cu4SO4(OH)6·2H2O

Timothy R. Dabinett, David Humberstone, Peter Leverett and Peter A. Williams

School of Natural Sciences, University of Western Sydney, Locked Bag 1797, Penrith South DC NSW 1797, Australia

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.