Mineral trapping through the precipitation of carbonate minerals is a potential approach to reduce CO2 accumulation in the atmosphere. The temperature dependence of amorphous magnesium carbonate (AMC), a precursor of crystalline magnesium carbonate hydrates, was investigated using synchrotron X-ray scattering experiments with atomic pair distribution function (PDF) and X-ray absorption fine structure analysis. PDF analysis revealed that there were no substantial structural differences among the AMC samples synthesized at 20, 60, and 80 °C. In addition, the medium-range order of all three AMC samples was very similar to that of hydromagnesite. Stirring in aqueous solution at room temperature caused the AMC sample to hydrate immediately and form a three-dimensional hydrogen-bonding network. Consequently, it crystallized with the long-range structural order of nesquehonite. The Mg K-edge X-ray absorption near-edge structure spectrum of AMC prepared at 20 °C was very similar to that of nesquehonite, implying that the electronic structure and coordination geometry of Mg atoms in AMC synthesized at 20 °C are highly similar to those in nesquehonite. Therefore, the short-range order (coordination environment) around the Mg atoms was slightly modified with temperature, but the medium-range order of AMC remained unchanged between 20 and 80 °C.

Neutron diffraction, Raman spectroscopy, and thermal analysis were performed to investigate the composition, structure, and formation conditions of the magnesium carbonate hydrate nesquehonite. The crystal structure of deuterated nesquehonite was analyzed by Rietveld refinement of the time-of-flight neutron powder diffraction pattern. The crystal structure possessed the monoclinic space group P2 1 /n with lattice parameters of a = 7.72100(12) Å, b = 5.37518(7) Å, c = 12.1430(3) Å, β = 90.165(4)°, and V = 503.956(13) Å 3 . The refinement with a final crystal structure model of deuterated nesquehonite converged to wRp = 4.22% and Rp = 3.50%. The result of structure refinement showed that two deuterium atoms are coordinated to the O1, O2, and O6 atoms as a water molecule in the nesquehonite. The fact that the three water molecules were included in the structure suggests the structural formula of the nesquehonite obtained in the study should be written as MgCO 3 •3H 2 O not Mg(HCO 3 )(OH)•2H 2 O.

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We investigated the structure changes and phase transformation from nanocrystalline mackinawite to pyrite using hydrothermal experiments, synchrotron X-ray diffraction (XRD) technique, atomic Pair Distribution Function (PDF) method, Extended X-ray Absorption Fine Structure (EXAFS) analysis, and transmission electron microscopic (TEM) observation. The first hydrothermal ageing experiment was performed by heating the nanocrystalline mackinawite at 120°C for 12 h. The nanocrystalline mackinawite remained essentially unchanged for 12 h. The d 001 and FWHM values of XRD peaks decreased for the first 2 h and subsequently maintained almost constant. There was no linear relationship between lattice parameters and hydrothermal heating time. The crystallite size quickly increased by the heating of 2 h, leading to the increase of crystallinity and appearance of the medium-range order in the nanocrystalline mackinawite. The nanocrystalline mackinawite preferentially grew in the horizontal direction along the sheet structure. The Fe atoms were distributed in the tetrahedral sites with a site occupancy of approximately 80%. The pre-edge peak energy of Fe K-edge suggested that about 10% Fe 3+ was included in the nanocrystalline mackinawite to compensate the charge deficiency of Fe 2+. The second hydrothermal ageing experiment was performed by heating the nanocrystalline mackinawite at 120°C under the presence of elemental sulfur for 24 h. The nanocrystalline mackinawite persisted up to 8 h of heating time. Thereafter, pyrite and greigite instead of the nanocrystalline mackinawite appeared. Finally pyrite became dominant. The d 001 and lattice parameters of nanocrystalline mackinawite varied significantly compared with those heated under the absence of elemental sulfur. The pre-edge peak energy indicated that the Fe 2+ was oxidized to Fe 3+ by elemental sulfur acting as the oxidant during the phase transformation from nanocrystalline mackinawite to greigite. In the phase transformation to pyrite, on the other hand, the Fe 3+ was reduced to Fe 2+ by sulfur in mackinawite and greigite acting as the reductant. The EXAFS analysis revealed that the second peak from the Fe-Fe interaction appeared at the heating time of 2 h, implying the formation of sheet structure consisting of edge-sharing FeS 4 tetrahedra. Intensity of the second shell peak from the Fe-Fe interaction reduced after the heating time of 8 h. Instead, new peaks corresponding to the Fe-S and Fe-Fe interaction appeared after the heating time of 12 h. This result was strongly associated with formation of the disulfide bonds (S-S bonds) in pyrite. Consequently, the elemental sulfur can be recognized as one of the most important factors to promote the phase transformation from mackinawite to pyrite in the reducing lake and marine sediments.