The classical terrace-ledge-kink model of crystal growth is widely used to interpret mineral formation in biological and geological systems. A key assumption underlying application of the model is that thermal fluctuations of steps are sufficiently rapid to produce an abundance of kink sites for attachment of growth units. High-resolution in situ atomic force microscopy (AFM) studies and kinetic Monte Carlo simulations of step-edge structure and dynamics show this physical picture to be invalid for the common mineral calcite whose steps exhibit low kink density and weak step edge fluctuations. As a consequence, interactions of impurities with calcite step edges cannot be interpreted with traditional thermodynamic models based on minimization of the Gibbs free energy. Instead, impurity-step interactions follow a different mechanism determined by the kinetics of attachment and detachment.Step advance is unimpeded when the creation of new kinks by attachment of growth units to the step outpaces binding of impurities to the newly created kinks. This kink-limited model offers a plausible explanation for reports of "kinetic disequilibrium" of trace element signatures. Moreover, because kink density is tied to crystal solubility, these findings argue for a theory based on weak fluctuations to interpret growth of many common crystalline phases of importance in geochemical, biological, and technological settings.

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Previous work has shown uranium (U) isotope fractionation between natural ferromanganese crusts and seawater. Understanding the mechanism that causes (238)U/(235)U fractionation during adsorption to ferromanganese oxides is a critical step in the utilization of (238)U/(235)U as a tracer of U adsorption reactions in groundwater as well as a potential marine paleoredox proxy. We conducted U adsorption experiments using synthetic K-birnessite and U-bearing solutions. These experiments revealed a fractionation matching that observed between seawater and natural ferromanganese sediments: adsorbed U is isotopically lighter by ∼0.2‰ (δ(238/235)U) than dissolved U. As the redox state of U does not change during adsorption, a difference in the coordination environment between dissolved and adsorbed U is likely responsible for this effect. To test this hypothesis, we analyzed U adsorbed to K-birnessite in our experimental study using extended X-ray absorption fine structure (EXAFS) spectroscopy, to obtain information about U coordination in the adsorbed complex. Comparison of our EXAFS spectra with those for aqueous U species reveals subtle, but important, differences in the U-O coordination shell between dissolved and adsorbed U. We hypothesize that these differences are responsible for the fractionation observed in our experiments as well as for some U isotope variations in nature.

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