IMPURITY INCORPORATION MECHANISM DRIVING CALCITE CRYSTAL GROWTH: IN SITU MEASUREMENTS COUPLED WITH MULTISCALE CHEMICAL IMAGING (Invited Presentation)

Impurity incorporation in calcite has important environmental implications, as the reactive surface structure of calcite influences the mobility and distribution of impurities. Although it's believed that lattice strain affects reaction rates, the underlying mechanisms remain poorly understood. In this study, we aim to determine the impurity incorporation mechanism by studying the kinetics of single-crystal calcite growth under varying [Sr]/[Ca]_aq using in situ flow-through atomic force microscopy (AFM). Observations showed that Sr inhibited the hillock growth and exerted a more pronounced suppression on one side of the hillock, the side where steps make an obtuse angle to the surface, whereas the opposite side, acute, showed fewer effects.

Additionally, we performed further analysis on the chemical composition and lattice strain of Sr-rich calcite through high-resolution chemical imaging techniques and nanoscale strain mapping. Using SEM, STEM-EDS, AFM time-of-flight secondary ion mass spectrometry (AFM-ToF-SIMS), and atom probe tomography (APT), the elemental composition of the growth hillock was excavated post-hoc with submicron resolution in 3D. The extent of Sr incorporation in the calcite growth hillock was sensitive to the Sr/Ca ratio in solution, showing that 80% of Sr in solution at 0-0.1 mM can be incorporated into calcite solid. The distribution of Sr was inhomogeneous on the obtuse side, whereas more evenly distributed on the acute side.

Moreover, the differences in Sr concentration between obtuse and acute sides also led to variations in the elastic modulus and strain. Specifically, for calcite grown at [Sr]/[Ca]_aq=0.19, obtuse side showed a 22% higher Sr compared to the acute side, resulting in a higher elastic modulus of ~2 GPa using contact resonance AFM (CRAFM). Raman mapping demonstrated higher energy vibrational modes on the obtuse side compared to acute side, suggesting that bonds were stronger due to Sr replacing Ca. Our 4D-STEM analysis revealed that the presence of Sr caused higher strain in the lattice compared to Sr-free calcite, and the increased strain further inhibited the subsequent growth of calcite by restricting the attachment of Ca atoms to the lattice. Overall, our study provides compelling evidence for a negative correlation between the crystal growth rate and lattice strain resulting from Sr incorporation.