Abstract
The speciation of calcium in chloride solutions has been investigated between 100 and 360°C by measuring the solubility of AgCl in HCl-CaCl 2 solutions in which chloride varies from 0.3 to 3.0 m and calcium is maintained constant at 0.1 m. Cumulative equilibrium formation constants of calcium chloride ion pairs were evaluated using a non-linear least squares procedure. Association constants could not be evaluated for calcium chloride ion pairs from the data at 100°C. However, at 150°C the cumulative formation constants for CaCl + and CaCl 2 0 are 0.85 and 1.73, respectively. The stability field for CaCl + decreases with increasing temperature, whereas that for CaCl 2 0 increases sharply and at 360°C K 2 is 4.95 · 10 4. Higher order calcium chloride ion pairs either do not form or have stability fields too small to be detected by the methods used in this study. The neutral aqueous calcium chloride ion pair CaCl 2 0 contributes significantly to calcium speciation in intermediate to high salinity hydrothermal solutions: at 250°C, 50 mol percent of the calcium in a 1 m HCl solution occurs as CaCl 2 0. The effect of this ion pairing is to increase the pH stability limits and solubilities of calcium-bearing minerals in such solutions.
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