Nanoscale Effects of Strontium on Calcite Growth: A Baseline for Understanding Biomineralization in the Absence of Vital Effects
Wilson, Darren Scott
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This study uses in situ atomic force microscopy (AFM) to directly observe the atomic scale effects of Sr on the monomolecular layer growth of abiotic calcite. These insights are coupled with quantitative measurements of the kinetics and thermodynamics of growth to determine the direction-specific effects of Sr on the positive and negative surface coordination environments that characterize calcite step edges. Low concentrations of strontium enhance calcite growth rate through changes in kinetics. A new conceptual model is introduced to explain this behavior. Higher concentrations of strontium inhibit and ultimately stop calcite growth by a step blocking mechanism. The critical supersaturation required to initiate growth (sigma*) increases with increasing levels of strontium. At higher supersaturations, strontium causes growth rates to increase to levels greater than those for the pure system. The step blocking model proposed by Cabrera and Vermilyea in 1958 does not predict the experimental data reported in this study because the dependence of sigma* upon strontium concentration is not the same for all supersaturations. Strontium inhibits calcite growth by different mechanisms for positive and negative step directions. Preliminary evidence indicates that strontium is preferentially incorporated into the positive step directions suggesting that impurity concentrations are not homogeneous throughout the crystal structure. Despite geochemical similarities, this study demonstrates that strontium and magnesium have different surface interaction mechanisms. The findings of this study demonstrate the importance of understanding microscopic processes and the significance of interpreting biominerals trace element signatures in the context of direction-specific interactions.
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