The incorporation of Mg 2+ and Sr 2+ into calcite overgrowths: influences of growth rate and solution composition

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The concentrations of Mg 2+ and Sr 2+ incorporated within calcite overgrowths precipitated from seawater and related solutions, determined at 25°C, were independent of the precipitation rate over approximately an order of magnitude. The saturation states used to produce this range of precipitation rates varied from 3 to 17 depending on the composition of the solution. The amount of Mg 2+ incorporated in the overgrowths was not directly proportional to Mg 2+ /Ca 2+ in solution over the entire range (1-20) of ratios studied. Below a ratio of 7.5, the overgrowth was enriched in MgCO 3 relative to what is predicted by the constant distribution coefficient measured above a ratio of 7.5. This increased MgCO 3 correlates with the relative enrichment of adsorbed Mg 2+ . Above a ratio of 7.5 the concentration of MgCO 3 in the calcite overgrowths followed a classical thermodynamic behavior characterized by a constant distribution coefficient of 0.0123 (±0.008 std dev). The concentration of SrCO 3 incorporated in the overgrowths was linearly related to the MgCO 3 content of the overgrowths, and is attributed to increased solubility of SrCO 3 in calcite due to the incorporation of the smaller Mg 2+ ions. The kinetic data indicate that the growth mechanism involves the adsorption of the cations on the surface of the calcite prior to dehydration and final incorporation. It is suggested that dehydration of cations at the surface is the rate controlling step.

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