The isotopic composition and chemical impurity content of natural calcite, aragonite and phosphates are widely used as indicators of the environment of formation of the minerals. Until recently, most of the observed effects have been interpreted in terms of equilibrium models. With the advent of precise measurements of Ca and Mg isotopic ratios it has now become clearer what the magnitude of non-equilibrium (kinetic) effects are, but the lack of a theory relating the isotopic and trace element compositions to crystal growth rates and environmental conditions made interpretation difficult. Over the past five years we have developed a conceptual-theoretical framework, supported by experiments, that begins to provide predictive understanding of the magnitudes and roles of kinetic and equilibrium isotopic and trace element fractionation in the formation of solids like CaCO<sub>3</sub> by precipitation from aqueous solutions. The particular value of isotopic and trace element data is to elucidate the magnitude of chemical exchange at mineral surfaces during growth, isolate the roles of desolvation and diffusion in controlling growth, and evaluate the interplay of impurity incorporation and growth rate. There is still a paucity of experimental data available for testing models, but there is ample evidence that kinetic isotopic and chemical effects can be valuable contributors to understanding precipitation reactions and for sensing crystal growth conditions for natural samples. Kinetic effects are so predominant at room temperature that it is difficult to establish equilibrium fractionation factors, which are also needed to test models. Reliable models for kinetic isotope and trace element fractionation, when added to reactive transport models, should also allow for new insights into the controls on carbonate mineral growth under natural conditions.

Isotopes and impurities as microprobes of the mineral surface dynamics of calcite growth

251st National Meeting (2016)

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