Dissolution and solubility of calcite-rhodochrosite solid solutions [(Ca_1-xMn_x)CO₃] at 25 °C

A complete series of calcite-rhodochrosite solid solutions [(Ca_1-xMn_x)CO₃] are prepared, and their dissolution processes in various water samples are experimentally investigated. The crystal morphologies of the solid solutions vary from blocky spherical crystal aggregates to smaller spheres with an increasing incorporation of Mn in the solids. Regarding dissolution in N₂-degassed water, air-saturated water and CO₂-saturated water at 25 °C, the aqueous Ca and Mn concentrations reach their highest values after 1240-2400 h, 6-12 h and < 1 h, respectively, and then decrease gradually to a steady state; additionally, the ion activity products (log_IAP) at the final steady state (≈ solubility products in log_K_sp) are estimated to be - 8.46 ± 0.06, - 8.44 ± 0.10 and - 8.59 ± 0.10 for calcite [CaCO₃], respectively, and - 10.25 ± 0.08, - 10.26 ± 0.10 and - 10.28 ± 0.03, for rhodochrosite [MnCO₃], respectively. As X_Mn increases, the log_IAP values decrease from - 8.44 ~ - 8.59 for calcite to - 10.25 ~ - 10.28 for rhodochrosite. The aqueous Mn concentrations increase with an increasing Mn/(Ca + Mn) molar ratio (X_Mn) of the (Ca_1-xMn_x)CO₃ solid solutions, while the aqueous Ca concentrations show the highest values at X_Mn = 0.53-0.63. In the constructed Lippmann diagram of subregular (Ca_1-xMn_x)CO₃ solid solutions, the solids dissolve incongruently, and the data points of the aqueous solutions move progressively up to the Lippmann solutus curve and then along the solutus curve or saturation curve of pure MnCO₃ to the Mn-poor side. The microcrystalline cores of the spherical crystal aggregates are preferentially dissolved to form core hollows while simultaneously precipitating Mn-rich hexagonal prisms.