Incorporation of Incompatible Strontium and Barium Ions into Calcite (CaCO3) through Amorphous Calcium Carbonate

Minerals

Jun Kawano¹  Daisuke Enomoto²  Hiroyuki Kagi²  Shiho Marugata²  Ayaka Saito²  Kazuki Komatsu²  Koji Maruyama² 

[1] Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan;Geochemical Research Center, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan;

关键词: calcite;    amorphous calcium carbonate;    disorder;    md simulation;   

DOI  :  10.3390/min10030270

来源: DOAJ

【 摘 要 】

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.

【 授权许可】

Unknown