STABILITY OF MANGANESE(II) CHLORIDE COMPLEXES FROM 25 TO 300-DEGREES-C

The stability and stoichiometry of the Mn(II) chloride complexes have been experimentally determined in the temperature range 25 to 300 degr ees C. The solubility of AgCl(s) was measured in solutions of fixed HC l concentration (0.01-6.0 m) and varying Sigma Mn/Sigma Cl molar ratio (0.0-0.5), following a modification of the method of Ruaya and Seward (1986). The results were regressed to obtain the following smoothed v alues for the first and second cumulative association constants (beta( 1) and beta(2), respectively)for the Mn(II) chloride complexes: [GRAPH ICS] Our study indicates that Mn2+ is the dominant species at 25 degre es C, whereas the stabilities of MnCl+ and MnCl20 increase rapidly wit h temperature. In the absence of competing Ligands, the neutral MnCl20 complex should dominate Mn transport for chloride-rich hydrothermal f luids at or above 300 degrees C. No evidence was found for complexes o f higher ligand number (e.g., MnCl3-, MnCl42-). Our results indicate t hat Mn is more strongly complexed than Fe at elevated temperature. The solubilities of some common manganese-bearing minerals have been calc ulated as a function of pH, a(Cl)(-), Sigma CO3, and temperature. The minerals rhodonite and rhodocrosite are highly soluble in mildly acidi c solutions. Most active geothermal systems are, therefore, expected t o be undersaturated with respect to these phases, although they may lo cally precipitate in response to sudden increases in pH (e.g., during boiling). Mn-silicate minerals will tend to dissolve in solutions unde rgoing cooling, whereas rhodocrosite exhibits either prograde or retro grade solubility, depending on salinity. Manganese mobilized by hydrot hermal fluids may travel considerable distances until contact is ultim ately made with an oxidized environment, at which point Mn-oxide miner als will precipitate.