Effect of mineral growth rate on Zinc incorporation into calcite and aragonite

The distribution coefficient of Zn²⁺ (i.e., D^⁎_Zn²⁺) between calcite/aragonite and reactive fluids were estimated as a function of mineral growth rate (10^–8.6 < r_p < 10^–7.3 mol/m²/s) at 25 °C and 1 bar pCO₂. The obtained results suggest that rate strongly influences D^⁎_Zn²⁺ in both minerals and the dependence of D^⁎_Zn²⁺ on growth rate can be described as: Log D^⁎_Zn²⁺_{, calcite} = −0.331 (±0.048) Log r_p – 0.991 (±0.375); R² = 0.82 Log D^⁎_Zn²⁺_{, aragonite} = 0.751 (±0.089) Log r_p + 5.160 (±0.702); R² = 0.88 These correlations for calcite and aragonite are in agreement with the incorporation of foreign ions in compatible and incompatible host mineral structures, respectively. The use of the surface reaction kinetic model (SRKM) to fit the experimental data provided good results for calcite. In contrast, SRKM was not able to provide a good fit for Zn incorporation in aragonite, underlying the difficulties to model incorporation of foreign elements incompatible with the host mineral structure. Moreover, a linear correlation was found between D^⁎_Zn²⁺ and the saturation degree of the reactive fluid with respect to the growing mineral phase that can be described as: Log D^⁎_Zn²⁺_calcite = −1.280 (±0.221) SI_calcite + 1.864 (±0.053); R² = 0.77 Log D^⁎_Zn²⁺_aragonite = 2.852 (±0.418) SI_aragonite - 1.640 (±0.138); R² = 0.82 Considering that SI_mineral = 0 reflects chemical equilibrium in the above equations, a Log D^⁎_Zn²⁺ at equilibrium was calculated for both calcite (1.9) and aragonite (−1.6). This value is in good agreement with the theoretical models for calcite, whereas in the case of aragonite it is 1.5 to 2.1 orders of magnitude lower than the previously published values. While the incorporation of Zn ions into calcite implies the formation of a dilute solid-solution between calcite and smithsonite (i.e. ZnCO₃), the incorporation of Zn ions into aragonite is likely associated to the density of defect sites at the growing mineral surface.