Thermodynamics of the basic copper sulfates antlerite, posnjakite, and brochantite

Weathering of primary copper sulfides leads to the formation of copper sulfate minerals, among them antlerite, Cu-3(SO4)(OH)(4), brochantite, Cu-4(SO4)(OH)(6), and posnjakite, Cu4SO4(OH)(6)center dot H2O. In this work, we performed a series of experiments to determine the stability of these phases. First, batches of 0.001 M CuSO4 solution were titrated with 0.1 M NaOH at temperatures between 20 and 85 degrees C to various end-point pH values. The phases obtained were brochantite, posnjakite, or tenorite (CuO), or mixtures thereof. From these samples, the well-crystallized brochantite and posnjakite samples, plus an antlerite sample synthesized in a separate way, were investigated by calorimetric techniques. The enthalpies of formation determined by acid-solution calorimetry in 5 N HCl for antlerite, brochantite, and posnjakite are -1733.6 +/- 5.3, -2194.4 +/- 7.0, and -2468.2 +/- 7.0 kJ mol(-1), respectively. The standard entropies retrieved from heat capacity data (measured by relaxation calorimetry) are 274.3 +/- 2.7, 343.1 +/- 3.4, 390.9 +/- 3.9 J mol(-1) K-1, respectively. The calculated Gibbs free energies of the three phases are -1453.6 +/- 5.4, -1824.9 +/- 7.1, and -2043.4 +/- 7.1 kJ mol(-1), respectively. We have reviewed all available thermodynamic data for the three basic copper sulfates, compared them to our results, and found a rough agreement. Of the phases considered here, posnjakite has no stability field but is always moderately metastable with respect to brochantite by +4.7 kJ mol(-1) (assuming water activity of unity). Brochantite is predicted to precipitate under mildly acidic and oxidizing conditions at low Cu(II) and S(VI) aqueous activities. Antlerite forms if the activity of Cu(II) is higher or if temperature is elevated. Tenorite will crystallize under neutral or basic conditions. Our data can describe the titration experiments and the phase relationships between the copper sulfates well. (C) 2013 Elsevier GmbH. All rights reserved.