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.