Complexation of Cu + in Hydrothermal NaCl Brines: Ab initio molecular dynamics and energetics

Abstract

Chloride complexation of Cu + controls the solubility of copper(I) oxide and sulfide ore minerals in hydrothermal and diagenetic fluids. Solubility measurements and optical spectra of high temperature CuCl solutions have been interpreted as indicating the formation of CuCl, CuCl 2 - , CuCl 3 2 - and CuCl 4 3 - complexes. However, no other monovalent cation forms tri- and tetrachloro complexes. EXAFS spectra of high temperature Cu-Cl solutions, moreover, appear to show only CuCl and CuCl 2 - complexes at T > 100 °C. To reconcile these results, I investigated the nature and stability of Cu-Cl complexes using ab initio cluster calculations and ab initio (Car–Parrinello) molecular dynamics simulations for CuCl–NaCl–H 2O systems at 25 to 450 °C. Ab initio molecular dynamic simulations of 1 m CuCl in a 4 m Cl solution give a stable CuCl 3 - complex at 25 °C over 4 ps but show that the third Cl − is weakly bound. When the temperature is increased along the liquid–vapour saturation curve to 125 °C, the CuCl 3 2 - complex dissociates into CuCl 2 - and Cl −; only CuCl 2 - forms at 325 °C and 1 kbar. Even in a 15.6 m Cl brine at 450 °C, only the CuCl 2 - complex forms over a 4 ps simulation run. Cluster calculations with a static dielectric continuum solvation field (COSMO) were used in an attempt directly estimate free energies of complex formation in aqueous solution. Consistent with the MD simulations, the CuCl 3 2 - complex is slightly stable at 25 °C but decreases in stability with decreasing dielectric constant ( ε). The CuCl 4 2 - complex is predicted to be unstable at 25 °C and becomes increasingly unstable with decreasing dielectric constant. In hydrothermal fluids ( ε < 30) both the CuCl 3 2 - and CuCl 4 3 - complexes are unstable to dissociation into CuCl 2 - and Cl −. The results obtained here are at odds with recent equations of state that predict CuCl 3 2 - and CuCl 4 3 - complexes are the predominant species in hydrothermal brines. In contrast, I predict that only CuCl 2 - complexes will be significant at T > 125 °C, even in NaCl-saturated brines. The high-temperature ( T > 125 °C) optical spectra of CuCl solutions and solubility measurements of Cu minerals in Cl-brines need to be reinterpreted in terms of only the CuCl and CuCl 2 - complexes.