Resolving the Control of Magnesium on Calcite Growth: Thermodynamic and Kinetic Consequences of Impurity Incorporation f
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Resolving the Control of Magnesium on Calcite Growth: Thermodynamic and Kinetic Consequences of Impurity Incorporation for Biomineral Formation
Kevin J. Davis, Patricia M. Dove and James J. De Yoreo* School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332 *Department of Chemistry and Materials Science, Lawrence Livermore National Laboratory, Livermore, CA 94550 ABSTRACT Magnesium is a key determinant in CaCO3 biomineral formation and has recently emerged as an important paleotemperature proxy. Atomic force microscopy (AFM) was used to determine the fundamental thermodynamic and kinetic controls of Mg2+ on calcite morphology and growth. Comparison of directly measured monomolecular step velocities (vs±) to theoretical crystal growth impurity models demonstrated calcite inhibition due to enhanced mineral solubility through Mg2+ incorporation. Terrace width (λ) measurements independently supported an incorporation mechanism by indicating a shift in the effective supersaturation (σeff) of the growth solutions in the presence of Mg2+. This study resolves the controversy over the molecular-scale mechanism of calcite inhibition by Mg2+ and provides an unambiguous model for the thermodynamic and kinetic consequences of impurity incorporation into CaCO3 biominerals.
INTRODUCTION The complex processes of biomineralization offer exciting prospects for the development of novel materials based upon biomimetic strategies [1-3]. In addition, CaCO3 biomineral formation has widespread implications for global biogeochemical cycles. Biogenic carbonate sediments are recognized as biogeochemically-significant due to their role in mediating ocean chemistry and atmospheric CO2 concentrations. Further, the trace element composition of CaCO3 biominerals has been shown to reflect the chemical and physical environments in which they formed, resulting in their use as important tools in paleoclimate determination [4]. Biomineralization occurs in the complex aqueous systems that are characteristic of natural environments. A critical step in understanding biomineral formation is to determine the fundamental interactions of common inorganic aqueous species with the growing mineral surface. Due to its ubiquity in natural waters, Mg2+ is the principal modifier of CaCO3 morphology and growth in biogeochemical environments [5]. More importantly, the presence of magnesium in calcium carbonate biominerals has been identified as an invaluable paleothermometer that is less susceptible to changes in salinity and polar ice volume than other proxies [6-9]. However, despite the contemporary need to understand paleoclimates, the physical basis by which Mg2+ modifies carbonate growth has yet to be discerned. In fact, the molecular-scale mechanism by which Mg2+ inhibits calcite continues to be the source of
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considerable controversy in the geochemical community. This disagreement is driven by conclusions from bulk studies that attribute calcite growth inhibition to either step-blocking by Mg2+ adsorption and slow-d
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