Dissolution and Solubility of the Solid Solution between Calcite and Smithsonite [(Ca1-x Zn x )CO3] at 25 & DEG;C

Thenon-ideal solid solutions between calcite and smithsonite,[(Ca1-x Zn x )CO3], were synthesized, and their interaction withdifferent aqueous solutions at 25 & DEG;C was experimentally investigated.The X-ray diffraction spectra indicated that all synthesized mineralsexhibited the calcite structure exclusively. After 180-240days of dissolution in N-2-degassed water (NDW) and air-saturatedwater (ASW), the aqueous Zn concentrations reached a constant valueranging from 0.002565 to 0.006133 and 0.002710 to 0.006374 mmol/Lfor the solid solutions with low Zn/(Zn + Ca) mole ratios (X-Zn < 0.075) or from 0.005416 to 0.076400 and 0.005128 to 0.067222mmol/L for the solid solutions with high X-Zn (>0.864),respectively. After 180-240 days of dissolution in CO2-saturated water (CSW), the aqueous Zn concentrations reached a constantvalue ranging from 0.005938 to 0.081753 mmol/L for all solid solutions.The aqueous Zn/(Ca + Zn) mole ratios were considerably lower thanthe solid X-Zn. The aqueous Zn and Ca concentrations generallyincreased with increasing X-Zn for solid solutions withX(Zn) < 0.075, while they decreased with increasing X-Zn for solid solutions with X-Zn > 0.864. Theaveragesolubility products (K (sp)) (& AP; ionactivity products at the constant state) were determined to be 10(-8.36 & PLUSMN;0.10), 10(-8.33 & PLUSMN;0.03), and 10(-8.28 & PLUSMN;0.06) for calcite [CaCO3] in NDW, ASW, and CSW, respectively. Similarly, the average solubilityproducts were determined to be 10(-10.65 & PLUSMN;0.12), 10(-10.60 & PLUSMN;0.08), and 10(-10.47 & PLUSMN;0.06) for smithsonite [ZnCO3] in NDW, ASW, and CSW, respectively.The logarithm of K (sp) showed a slight increasewith increasing X-Zn for solid solutions with X-Zn < 0.075, whereas it decreased with increasing X-Zn forthe solids with X-Zn > 0.864. In the Lippmann diagramconstructedwith the Guggenheim coefficients a (0) =2.72 and a (1) = -0.266 for the [(Ca1-x Zn x )CO3] solid solutions, it was observed that the solid solutionsdissolved non-stoichiometrically and moved progressively up to theminimum stoichiometric saturation curve for pure smithsonite and the solutus curve and then along them from right to left, finallyreaching the saturation curve for calcite. The coexistence of Zn-pooraqueous solutions with ZnCO3-rich solids highlights thefindings of the [(Ca1-x Zn x )CO3] mineral-water reaction andits significance in the zinc geochemical cycle in Earth's surfaceenvironments, contributing to a comprehensive understanding of theseprocesses.