Incorporation of Incompatible Strontium and Barium Ions into Calcite (CaCO3) through Amorphous Calcium Carbonate
Ayaka Saito, Hiroyuki Kagi, Shiho Marugata, Kazuki Komatsu, Daisuke Enomoto, Koji Maruyama, Jun Kawano
Abstract
Calcite is a ubiquitous mineral in nature. Heavy alkaline-earth elements with large ionic radii such as Sr2+ and Ba2+ are highly incompatible to calcite. Our previous study clarified that incompatible Sr2+ ions can be structurally incorporated into calcite through crystallization from amorphous calcium carbonate (ACC). In this study, we synthesized Sr-doped calcite with Sr/(Sr + Ca) up to 30.7 ± 0.6 mol% and Ba-doped calcite with Ba/(Ba + Ca) up to 68.6 ± 1.8 mol%. The obtained Ba-doped calcite samples with Ba concentration higher than Ca can be interpreted as Ca-containing barium carbonates with the calcite structure which have not existed so far because barium carbonate takes the aragonite structure. X-ray diffraction (XRD) patterns of the Sr-doped and Ba-doped calcite samples obtained at room temperature showed that reflection 113 gradually weakened with increasing Sr/(Sr + Ca) or Ba/(Ba + Ca) ratios. The reflection 113 disappeared at Ba/(Ba + Ca) higher than 26.8 ± 1.6 mol%. Extinction of reflection 113 was reported for pure calcite at temperatures higher than 1240 K, which was attributed to the rotational (dynamic) disorder of CO32− in calcite. Our Molecular Dynamics (MD) simulation on Ba-doped calcite clarified that the CO32− ions in Ba-doped calcites are in the static disorder at room temperature. The CO32− ions are notable tilted and displaced from the equilibrium position of pure calcite.