[S-MP36] Physics and Chemistry of Minerals

[SMP36-05] Crystal structure of nesquehonite, MgCO₃・3(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) ų. 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.