Zinc Ions Modify Calcium Oxalate Growth by Distinct Transformation of Crystal Surface Termination

Calcium oxalate crystals are ubiquitous minerals in biogenic, geological, and synthetic systems. It has been shown that the most naturally abundant form of these crystals, calcium oxalate monohydrate (COM), grows via a classical pathway that can be regulated by crystal growth modifiers. One of the most important occurrences of COM is during human kidney stone disease where the role of zinc in pathological stone formation is not fully understood. There are conflicting claims in the literature that zinc functions either as a promoter or inhibitor of COM stone formation. Here, we examine the role of zinc ions in COM crystal growth using a combination of experimental and modeling techniques to elucidate ion–crystal interactions at macroscopic to atomic length scales. From this, we show that zinc reduces the rate of crystal growth and also induces morphological transformations via the introduction of intergrowths that can potentially accelerate crystal growth. Density functional theory calculations indicate high energetic barriers for zinc incorporation in COM crystals, suggesting that changes in crystal growth are related to zinc interactions with COM crystal surfaces. This was confirmed by in situ atomic force microscopy measurements showing a unique ability of zinc ions to truncate the height of layers on the (100) face, which concomitantly affects both the morphology of surface features and the kinetics of layered growth. Collectively, our study suggests that the role of zinc in pathological crystallization is potentially bimodal, consistent with opposing theories in the literature.