Mode of formation of some rare copper(II) and lead(II) minerals from aqueous solution, with particular reference to deposits at tiger, Arizona

Summary

Free energies of formation of the rare copper(II) secondary minerals linarite, (Pb,Cu)₂SO₄(OH)₂, caledonite, Pb₅Cu₂CO₃(SO₄)₃(OH)₆, and wherryite, Pb₄CuCO₃(SO₄)₂O(OH,Cl)₂, and of the complex lead(II) species leadhillite, Pb₄SO₄(CO₃)₂(OH)₂, have been estimated using solution techniques. Values derived are: −1212(1), −4328(2), −2871(1), and −2525(4) kJ mol⁻¹ at 298.2K respectively, and have been used to construct stability field diagrams involving these, and related species. They are anomalous in the sense that they are not the usually found sulphates and carbonates of copper(II) and lead(II) in the natural environment. The data, together with field observations of mineral associations, has been used to reconstruct part of the chemical paragenetic sequence of the oxidized zone of the Mammoth —St. Anthony Mine, Tiger, Arizona, U.S.A. Many of the rare species above and other complex halides of copper(II), silver(I) and lead(II) formed under conditions where$a_{H_2 CO_3^0 }$ and a_SO ²⁻₄ were comparatively low, and around neutral values of pH. Several trends in the chemistry of the development of the anomalous oxidized zone at Tiger are apparent, and these are discussed in the light of the above findings.