The solubility of nantokite (CuCl(s)) and the structure of the predominant copper species in supercritical water (290-400 bar Lit 420 degrees C; 350-450 degrees C at 290 bar; 500 degrees C at 350 bar; density = 0.14-0.65 g/cm(3)) were investigated concurrently using synchrotron X-ray absorption spectroscopy (XAS) techniques. These conditions were chosen as they represent single phase solutions near the critical isochore, where the fluid density is intermediate of typical values for vapour and brine and is highly sensitive to even small changes in pressure. X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption spectroscopy (EXAFS) analyses show that aqueous copper occurs in a slightly distorted linear coordination it) the solutions studied, with an average of 1.35(+/- 0.3) Cl and 0.65(+/- 0.3) O neighbours. The solubility of CuCl(s) decreases exponentially with decreasing water density (i.e., decreasing pressure at constant temperature), in a manner similar to the Solubility behaviour of salts such as NaG in water vapour. Based on this similarity, an apparent equilibrium constant for the dissolution reaction of 0.5 +/- 0.4 was calculated from a regression of the data at 420 degrees C, and it was determined that each Cu atom is solvated by approximately three water molecules. This indicates that under these conditions, copper Solubility is controlled mainly by the structure of the second-shell hydration, which is essentially invisible to the XAS techniques used in this study. These results demonstrate that for a supercritical fluid near the critical isochore. decreasing pressure may initiate precipitation of copper even before boiling or phase separation. Such a process could be responsible for near-surface ore deposition in seafloor hydrothermal systems, where supercritical fluids experience rapid pressure changes during the transition between lithostatic and hydrostatic domains. (C) 2008 Elsevier Ltd. All rights reserved.