Incorporation of Incompatible Strontium and Barium Ions into Calcite (CaCO3) through Amorphous Calcium Carbonate
by
Ayaka Saito 1, Hiroyuki Kagi 1,*, Shiho Marugata 1, Kazuki Komatsu 1, Daisuke Enomoto 1, Koji Maruyama 1 and Jun Kawano 2
Ayaka Saito 1, Hiroyuki Kagi 1,*, Shiho Marugata 1, Kazuki Komatsu 1, Daisuke Enomoto 1, Koji Maruyama 1 and Jun Kawano 2
1
Geochemical Research Center, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
2
Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(3), 270; https://doi.org/10.3390/min10030270
Received: 31 January 2020 / Revised: 9 March 2020 / Accepted: 13 March 2020 / Published: 17 March 2020
(This article belongs to the Special Issue Mineral Physics—In Memory of Orson Anderson)
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.
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Keywords:
calcite; amorphous calcium carbonate; disorder; MD simulation
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Saito, A.; Kagi, H.; Marugata, S.; Komatsu, K.; Enomoto, D.; Maruyama, K.; Kawano, J. Incorporation of Incompatible Strontium and Barium Ions into Calcite (CaCO3) through Amorphous Calcium Carbonate. Minerals 2020, 10, 270.
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