Dissolution and Solubility of the Solid Solution between Calcite and Smithsonite [(Ca_1–xZn_x)CO₃] at 25 °C

Abstract

The non-ideal solid solutions between calcite and smithsonite, [(Ca1–x Zn x )­CO3], were synthesized, and their interaction with different aqueous solutions at 25 °C was experimentally investigated. The X-ray diffraction spectra indicated that all synthesized minerals exhibited the calcite structure exclusively. After 180–240 days of dissolution in N2-degassed water (NDW) and air-saturated water (ASW), the aqueous Zn concentrations reached a constant value ranging from 0.002565 to 0.006133 and 0.002710 to 0.006374 mmol/L for the solid solutions with low Zn/(Zn + Ca) mole ratios (XZn < 0.075) or from 0.005416 to 0.076400 and 0.005128 to 0.067222 mmol/L for the solid solutions with high XZn (>0.864), respectively. After 180–240 days of dissolution in CO2-saturated water (CSW), the aqueous Zn concentrations reached a constant value ranging from 0.005938 to 0.081753 mmol/L for all solid solutions. The aqueous Zn/(Ca + Zn) mole ratios were considerably lower than the solid XZn. The aqueous Zn and Ca concentrations generally increased with increasing XZn for solid solutions with XZn < 0.075, while they decreased with increasing XZn for solid solutions with XZn > 0.864. The average solubility products (K sp) (≈ ion activity products at the constant state) were determined to be 10–8.36±0.10, 10–8.33±0.03, and 10–8.28±0.06 for calcite [CaCO3] in NDW, ASW, and CSW, respectively. Similarly, the average solubility products were determined to be 10–10.65±0.12, 10–10.60±0.08, and 10–10.47±0.06 for smithsonite [ZnCO3] in NDW, ASW, and CSW, respectively. The logarithm of K sp showed a slight increase with increasing XZn for solid solutions with XZn < 0.075, whereas it decreased with increasing XZn for the solids with XZn > 0.864. In the Lippmann diagram constructed with 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 solutions dissolved non-stoichiometrically and moved progressively up to the minimum stoichiometric saturation curve for pure smithsonite and the solutus curve and then along them from right to left, finally reaching the saturation curve for calcite. The coexistence of Zn-poor aqueous solutions with ZnCO3-rich solids highlights the findings of the [(Ca1–x Zn x )­CO3] mineral–water reaction and its significance in the zinc geochemical cycle in Earth’s surface environments, contributing to a comprehensive understanding of these processes.