Hydrothermal solubility of rhodochrosite, Mn (II) speciation, and equilibrium constants
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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 90°C 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.