HYDROTHERMAL SOLUBILITY OF RHODOCHROSITE, MN(II) SPECIATION, AND EQUILIBRIUM-CONSTANTS

The aqueous solubility of natural rhodochrosite (MnCO3) has been studi ed as a function of pH and total dissolved carbonate concentration, fr om 25 to 275 degrees C and at saturation pressures. At 25 and 90 degre es C the solubility measurements:were conducted in Al-capped glass via ls which were additionally encapsulated in CO2-filled Al cans to preve nt CO2 losses or air diffusion into the vials. At 200 and 275 degrees C the runs were performed in Ag tubes. Natural, very pure, and well-cr ystallized rhodochrosite from Hotazel, South Africa was employed in al l cases. Manganese concentrations were analyzed by AA and ICP-MS, carb onate was weighed in, and pH was measured at 25 and 90 degrees C and w as calculated at higher temperatures. The experimentally derived solub ility curves allowed identification of the principal Mn-species. With increasing pH, Mn2+, MnHCO3+, MnCO30, Mn(OH)CO3- (only above 25 degree s C), and Mn(OH)(2)(0) (only at low total carbonate) were found to be the dominating species. From the experimental dataset, solubility prod ucts and equilibrium constants for the dissolution reactions, as well as for complex formation and hydrolysis of the Mn2+ ion, were calculat ed. The equilibrium constants show that the Mn-carbonate species are m oderately strong complexes, and in view of the abundance of CO2 in man y geological fluids, carbonate complexing of Mn2+ may be important in neutral and alkaline solutions.