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.
Highlights
Calcium carbonate (CaCO3 ) is a ubiquitous mineral in nature that has three anhydrous polymorphs: calcite, aragonite, and vaterite
Calcite was observed as the crystalline phase after pressure treatment on the amorphous calcium carbonate (ACC) samples precipitated from solutions with Sr/(Sr + Ca) < 35 mol%
Strontianite (SrCO3 with the aragonite structure) was observed as a crystalline phase in addition to calcite after pressure treatment on ACC obtained from starting solutions with Sr/(Sr + Ca) > 40 mol%
Summary
Calcium carbonate (CaCO3 ) is a ubiquitous mineral in nature that has three anhydrous polymorphs: calcite, aragonite, and vaterite. Vaterite is a metastable phase and it is a rare mineral compared to calcite and aragonite In addition to these three anhydrous polymorphs, amorphous calcium carbonate (hereafter amorphous calcium carbonate (ACC); CaCO3 ·nH2 O, n < 1.5) is known to exist [1]. Divalent ions with an ionic radius smaller than that of Ca2+ (1.00 Å) form a carbonate with the calcite structure. Divalent ions with an ionic radius larger than that of Ca2+ form a carbonate with the aragonite structure. Carbonates with the calcite structure tend to capture impurity elements of which the ionic radius is smaller than that of calcium ion. Carbonates with the aragonite structure tend to capture impurity elements of which the ionic radius is larger than that of Ca2+. Changes in properties of Sr-doped and Ba-doped calcite were investigated as well
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
