Abstract
Calcite formation via an amorphous calcium carbonate (ACC) precursor phase potentially offers a method for enhanced incorporation of incompatible trace metals, including Sr2+. In batch crystallization experiments where CaCl2 was rapidly mixed with Na2CO3 solutions, the Sr2+/Me2+ ratio was varied from 0.001 to 0.1, and the pathway of calcite precipitation was directed by either the presence or absence of high Mg2+ concentrations (i.e., using a Mg2+/total Me2+ ratio of 0.1). In the Mg-free experiments crystallization proceeded via ACC → vaterite → calcite, and average Kd Sr values were between 0.44 and 0.74. At low Sr2+ concentrations (Sr2+/Me2+ ratio ≤ 0.01), extended X-ray absorption fine structure analysis revealed that the Sr2+ was incorporated into calcite in the 6-fold coordinate Ca2+ site. However, at higher Sr2+ concentrations (Sr2+/Me2+ ratio = 0.1), Sr2+ was incorporated into calcite in a 9-fold site with a local coordination similar to Ca2+ in aragonite, but calcite-like at longer distances (i.e....
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