gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1986.050.355.05 Mineral Chemistry Stability Constants and Formation of Cu(II) and Zn(II) Phosphate Minerals in the Oxidized Zone of Base Metal Orebodies Clara M. Magalhães F. de Jesus Julio Pedrosa Department of Chemistry, University of Aveiro, Aveiro, Portugal Williams Peter A. Department of Chemistry, University College, PO Box 78, Cardiff CF1 1XL, Wales, UK 3 1986 50 355 33 39 11 12 1984 2 5 1985 Copyright © 1986, The Mineralogical Society 1986 Abstract

The free energies of formation of the minerals libethenite, Cu2PO4OH, pseudomalachite, Cu5(PO4)2(OH)4, cornetite, Cu3PO4(OH)3, tarbuttite, Zn2PO4OH, spencerite, Zn2PO4OH · 1.5H2O, and scholzite, CaZn2(PO4)2 · 2H2O, have been determined at 298.2 K using solution techniques. The values of ΔfG° (298.2 K) for the above minerals are, respectively, −1228.8±3.0, −2840.3±4.2, −1600.9±5.9, −1632.1±4.0, −1982.4±3.1, and −3556.7±7.6 kJ mol−1. These results, together with others from the literature concerning hopeite, Zn3(PO4)2·4H2O, hydroxyapatite, Ca5(PO4)3(OH), and the synthetic salt Cu3(PO4)2 · 2H2O have been used to construct a model for the formation of suites of Zn(II) and Cu(II) phosphate minerals found in the oxidation zone of base metal orebodies. The distributions and modes of occurrence of many natural assemblages of these minerals are readily explained by the model. Particular attention has been focused on deposits at Broken Hill, Zambia, and Saginaw Hill, Arizona, USA.

 Keywords stability constants phosphate minerals libethenite pseudomalachite cornetite tarbuttite spencerite scholzite