Opposing Effects of Impurity Ion Sr²⁺on the Heterogeneous Nucleation and Growth of Barite (BaSO₄)

Nucleation of barite (BaSO₄) has broad implications in geological, environmental, and materials sciences. While impurity metals are common, our understanding of how they impact nucleation remains dim. Here, we used classical optical microscopy compared to fast X-ray nanotomography (XnT) to investigate heterogeneous nucleation of barite on silica in situ with Sr²⁺as an impurity ion. The observed barite nucleation rates were consistent with classical nucleation theory (CNT), where barite crystals displayed a nonuniform size distribution, exhibiting distinct morphologies and incubation periods in Sr-free solutions. While undetectable with optical microscopy, nanotomography revealed that addition of Sr²⁺enhances nucleation rates driven by the pre-factor in CNT, likely because both adsorbed Ba²⁺and Sr²⁺act as precursor sites on which nucleation occurs. Sr²⁺simultaneously inhibits growth, however, leading to a homogeneous distribution of smaller crystals. This finding will enable an improved predictive understanding of nucleation in natural and synthetic environments.