Dissolution of the smithsonite–rhodochrosite (ZnCO3-MnCO3) solid solutions in aqueous solution at 25 °C

A series of smithsonite-rhodochrosite solid solutions [(Zn1-xMnx)CO3] (XMn = 0.00–1.00) were prepared in lab to study their dissolution process in aqueous solution. The molar ratios Mn/(Zn + Mn) of the synthetic solids (XMn) were very close to that of the starting mixtures. The X-ray diffraction (XRD) patterns of the synthetic solids showed continuously peak shifting to lower 2θ value with the increasing of XMn. For the dissolution in the nitrogen-degassed and the air-saturated water, the Zn and Mn contents in aqueous phase went up quickly to a constant value; after dissolution, the weak (200) peak of Zn5(CO3)2(OH)6 (hydrozincite) emerged in the XRD patterns of the solids with XMn = 0.00–0.30. For the dissolution in the carbon-dioxide-saturated water, the Zn and Mn contents in water reached high values quickly after 48 h ~ 120 h, and then declined to a stable value. After 270 d of dissolution, the aqueous molar ratios Mn/(Zn + Mn) were greater than or close to XMn of the corresponding solids. For the dissolution in the nitrogen-degassed, the air-saturated and the carbon-dioxide-saturated waters at 25 °C, the mean log IAP values (ion activity product) at the equilibrium state (≈ solubility product log Ksp) were determined to be −10.56, −10.54 and − 10.71 for smithsonite, and − 10.29, −10.26 and − 10.18 for rhodochrosite, respectively. With the increasing of XMn, the log IAP values increased non-linearly from −10.54 ~ −10.71 to −10.18 ~ −10.29, while the ΔGf° decreased linearly from −735.08 to −736.04 kJ/mol and − 813.99 to −814.63 kJ/mol. The constructed Lippmann diagrams showed that the (Zn1-xMnx)CO3 solid solutions dissolved incongruently, and the dissolution data points moved progressively up and exceeded the solutus curve, and then move along the (Zn1-xMnx)CO3 saturation curves from left to right or right to left for the solids with lower XMn (0.10–0.30) and higher XMn (0.40–0.90), respectively.