JpGU-AGU Joint Meeting 2020

Presentation information

[E] Oral

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

[S-MP36] Physics and Chemistry of Minerals

convener:Seiji Kamada(Frontier Research Institute for Interdisciplinary Sciences, Tohoku University), Masahiro KAYAMA(Department of General Systems Studies, Graduate School of Arts and Sciences, The University of Tokyo)

[SMP36-05] Crystal structure of nesquehonite, MgCO33(H,D)2O by neutron diffraction, Raman spectroscopy, and thermal analysis

*Gen-ichiro Yamamoto1, Atsushi Kyono1, Jun Abe2, Asami Sano-Furukawa3, Takanori Hattori3 (1.University of Tsukuba, 2.CROSS, 3.JAEA)

Keywords:Nesquehonite, Magnesium carbonate hydrate, Neutron diffraction, Raman spectroscopy, Thermal analysis, Hydrogen bonding network

Neutron diffraction, Raman spectroscopy, and thermal analysis were performed to investigate the composition, structure, and formation conditions of magnesium carbonate hydrate, nesquehonite. Nesquehonite was prepared from a mixture of MgCl2 and Na2CO3 solution at pH 10.9. The crystal structure of deuterated nesquehonite was analyzed by Rietveld refinement of the time-of-flight neutron powder diffraction pattern. The crystal structure possessed monoclinic space group P21/n with lattice parameters of a = 7.72100(12), b = 5.37518(7), c = 12.1430(3) Å, β = 90.165(4) o, and V = 503.956(13) Å3. The refinement with a final crystal structure model of deuterated nesquehonite converged to wRp = 4.22 % and Rp = 3.50 % (Fig. 1). The positions of six deuterium (D) atoms were successfully determined. The result indicated that the D atoms were coordinated to O1, O2, and O6 atoms as water molecules in nesquehonite. It can be therefore considered that nesquehonite obtained in the study had the chemical formula of MgCO3・3D2O. The Raman bands corresponding to the CO3 bending and stretching vibrations and the OH(OD) stretching vibration agreed substantially with those given by previous studies. The OH stretching vibration at 3555 cm-1 was also well consistent with those reported in the literature. It was experimentally impossible to distinguish the difference in chemical formula between MgCO3・3H2O and Mg(HCO3)(OH)・2H2O by using the powder XRD pattern and the Raman bands of the OH stretching vibrations. The differential thermal analysis (DTA) curve showed three endothermic peaks corresponding to three dehydration reactions, which indicated that three water molecules were released step by step from the MgCO3・3H2O nesquehonite. There is a distinct difference in dehydration behavior between MgCO3・3H2O and Mg(HCO3)(OH)・2H2O. Therefore, the thermal analysis would be a most useful tool to distinguish the difference in chemical formula of nesquehonite.