The aqueous geochemistry of the rare-earth elements and yttrium: 2. Theoretical predictions of speciation in hydrothermal solutions to 350°C at saturation water vapor pressure

Abstract

Stability constants for trivalent REE complexes containing fluoride, chloride, sulfate, carbonate or hydroxide ions have been predicted up to 350°C at saturated water vapor pressure using Helgeson's electrostatic approach, combined with the isocoulombic approach and available experimental thermodynamic data at low temperatures. In addition, the equilibrium boundary between divalent and trivalent REE ions was calculated at elevated temperatures and 1 kbar for Sm, Yb and Eu as a function of pH and oxygen fugacity. Calculations suggest that the divalent state for all three REE (Sm, Yb and Eu) becomes increasingly important at geologically reasonable pH and oxygen fugacity conditions as temperature increases, but only Eu, and possibly Yb, will be present in the divalent state in most hydrothermal solutions at temperatures of <300°C. Nevertheless, at magmatic temperatures (>600°C) the divalent state of many of the REE should prevail over the trivalent state, especially at low pressures (<2 kbar). Extrapolation of the available low-temperature stability constant data indicates that the stabilities of all the trivalent REE complexes considered increase relatively rapidly with temperature, as expected for complexation between hard metal ions and hard ligands. The following table illustrates some of the predicted data for the first cumulative stability constants of sulfate, fluoride and chloride complexes, as well as the first hydrolysis constant for La 3+ and Lu 3+ at 25° and 300°C: The predicted increase in stability constants with temperature is greatest for fluoride and least for chloride. Even allowing for uncertainty in the estimated stability constants, at geologically reasonable ligand concentrations, fluoride complexes predominate over all others in the mildly acidic to mildly basic pH region at elevated temperatures. Because of the predicted high stability of fluoride and carbonate complexes of the trivalent REE, as well as the empirically observed correlation between REE mobility and the presence of these ligands (in the minerals fluorite and calcite) in many hydrothermally affected geological environments, accurate experimental determination of the stability constants of these complexes should be a priority.