Japan Geoscience Union Meeting 2019

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-MP Mineralogy & Petrology

[S-MP33] Physics and Chemistry of Minerals

Wed. May 29, 2019 9:00 AM - 10:30 AM A07 (TOKYO BAY MAKUHARI HALL)

convener:Seiji Kamada(Frontier Research Institute for Interdisciplinary Sciences, Tohoku University), Masahiro KAYAMA(Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University), Chairperson:Seiji Kamada(FRIS, Tohoku Univ.)

9:15 AM - 9:30 AM

[SMP33-02] Synthesis of barium carbonate with calcite structure and rotational disorder of carbonate ions

*Hiroyuki Kagi1, Ayaka Saito1, Kazuki Komatsu1, Jun Kawano2 (1.Geochemical Research Center, Graduate School of Science, University of Tokyo, 2.Fuculty of Science, Hokkaido University)

Keywords:barium carbonate, molecular dynamics simulation, rotational disorder, calcium carbonate (calcite)

Structures of divalent metal carbonate is constrained by the ionic radii of cations. Cations larger than Ca2+ take the aragonite structure and cations smaller than Ca2+ take the aragonite structure. Barium carbonate (witherite) takes the aragonite structure and Ba2+ is incompatible to calcite. We have reported that large incompatible cations can be introduced to calcite through amorphous calcium carbonate (ACC). In this study, we synthesized Ba0.7Ca0.3CO3 with the calcite structure by heating treatment on ACC containing Ba2+. The obtained crystal can be interpreted as Ca-containing barium carbonate with the calcite structure, which cannot be found in nature or synthesized from a direct precipitation from the supersaturated solutions.

X-ray diffraction (XRD) patterns of Ba-doped calcite measured at 300 K shows the extinction of 113 reflection with increasing Ba concentration (see Figure 1). The extinction of the 113 reflection was reported on pure calcite at temperatures higher than 1240 K (Ishizawa et al., 2013). The change was attributed to the rotational disorder of CO32- ions. In contrast, we have observed the extinction of 113 reflection on the Ba-doped calcite at 300 K.

To investigate the behavior of CO32- ions at an atomistic level, molecular dynamics (MD) simulation of Ba-doped calcite was conductedusing the MD program, MXDTRICL (Kawamura 1997). Intensity ratio of reflections 113 to 104 (I113/I104) was measured from XRD patterns obtained from experimental study and MD simulation. I113/I104from experiment dropped to 0 at Ba/(Ba + Ca) = 26.8 ± 1.6 mol% while that of MD-simulated Ba-doped calcite gradually weakened and did not fall to zero. MD simulations of 25.5% and 50.9% Ba-doped calcite at high temperature were also conducted. In 25.5% Ba-doped calcite, I113/I104 remarkably dropped at around 1050 K with the structure of rotationally disordered CO32- ions. The transition temperature to the rotational disorder was at around 850 K for 50.9% Ba-doped calcite. These simulated temperatures are much lower than the transition temperature (1250 K) obtained from MD simulation on pure calcite (Kawano et al., 2009). The simulated results showed that the transition temperature to the rotational disordered phase dramatically decreased with increasing Ba/(Ba + Ca) in calcite.