Crystal structure of nesquehonite, MgCO₃·3H(D)₂O by neutron diffraction and effect of pH on structural formulas of nesquehonite

Abstract

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 P2₁/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) ų. 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 MgCO₃·3H₂O not Mg(HCO₃)(OH)·2H₂O.