The Controlling Effect of CaCO3 Supersaturation over Zn Carbonate Assemblages: Co-Crystallization in Silica Hydrogel

Weathering products of sphalerite-bearing ores play an important role in controlling the fate of Zn in the environment. In this framework, the relative stability of Zn carbonates is of special relevance for the common case of ore weathering by carbonated groundwater in the presence of calcium carbonates. We investigated the experimental (co)nucleation and growth of Zn and Ca carbonates at 25 degrees C in finite double diffusion silica hydrogel media with the purpose of deciphering the system's reactive pathway and unraveling the major governing factors behind the obtained mineral assemblages. The crystallized solids were carefully extracted two months post-nucleation and studied with micro-Raman spectroscopy, micro X-ray diffraction (XRD), scanning electron microscopy, and electron microprobe (EMP) methods. The obtained results indicate that the grown Zn-bearing phases corresponded to smithsonite and/or Zn hydroxyl carbonate, while CaCO3 polymorphs aragonite and calcite were also crystallized. Moreover, the observed mineral textural relationships reflected the interplay between supersaturation with respect to CaCO3/pCO2 and the grown Zn-bearing carbonate. Experiments conducted in more supersaturated conditions with respect to CaCO3 polymorphs (higher pCO2) favored the precipitation of smithsonite, while the opposite was true for the obtained Zn hydroxyl carbonate phase. The gathered Raman, XRD, and EMP data indicate that the latter phase corresponded to a non-stoichiometric, poorly crystalline solid.