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

Abstract

The aqueous solubility of natural rhodochrosite (MnCO3) has been studied as a function of pH and total dissolved carbonate concentration, from 25 to 275°C and at saturation pressures. At 25 and 90oC the solubility measurements were conducted in Al-capped glass vials which were additionally encapsulated in CO2-filled Al cans to prevent CO2 losses or air diffusion into the vials. At 200 and 275°C the runs were performed in Ag tubes. Natural, very pure, and well-crystallized rhodochrosite from Hotazel, South Africa was employed in all cases. Manganese concentrations were analyzed by AA and ICP-MS, carbonate was weighed in, and pH was measured at 25 and 90°C and was calculated at higher temperatures. The experimentally derived solubility curves allowed identification of the principal Mn-species. With increasing pH, Mn2+, MnHCO+3, MnCO03, Mn(OH)CO-3 (only above 25°C), and Mn(OH)02 (only at low total carbonate) were found to be the dominating species.From the experimental dataset, solubility products and equilibrium constants for the dissolution reactions, as well as for complex formation and hydrolysis of the Mn2+ ion, were calculated. The equilibrium constants show that the Mn-carbonate species are moderately strong complexes, and in view of the abundance of CO2 in many geological fluids, carbonate complexing of Mn2+ may be important in neutral and alkaline solutions.