Crystallization and Characterization of a New Magnesium Sulfate Hydrate MgSO₄·11H₂O

The MgSO₄ crystal hydrate formed below approximately 0 °C was proven to be the undecahydrate, MgSO₄·11H₂O (meridianiite) instead of the reported dodecahydrate MgSO₄·12H₂O. The crystals were grown from solution by eutectic freeze and by cooling crystallization. The crystal structure analysis and the molecular arrangement of these crystals were determined using single crystal X-ray diffraction (XRD). Reflections were measured at a temperature of 110(2) K. The structure is triclinic with space group P1̅ (No. 2). The crystal is a colorless block with the following parameters F.W. = 318.55, 0.54 × 0.24 × 0.18 mm³, a = 6.72548(7) Å, b = 6.77937(14) Å, c = 17.2898(5) Å, α = 88.255(1)°, β = 89.478(2)°, γ = 62.598(1)°, V = 699.54(3) ų, Z = 2, D_calc = 1.512 g/cm³, μ = 0.343 mm⁻¹. Raman spectroscopy was used for characterizing MgSO₄·11H₂O and for comparing the vibrational spectra with the MgSO₄·7H₂O salt. Between the two salts, there are significant differences mainly in the type of interactions of water with sulfate groups in the lattice, in view of the different O−H stretching vibrations, as well as sulfate, O−H···O (sulfate) and O−Mg−O bands vibrational modes. Thermogravimetric analysis confirmed the stochiometry of the MgSO₄·11H₂O salt. Additionally, the Miller indices of the major faces of MgSO₄·11H₂O crystals were defined.