The MgSO4 crystal hydrate formed below approximately 0 degrees C was proven to be the undecahydrate, MgSO4 center dot 11H(2)O (meridianiite) instead of the reported dodecahydrate MgSO4 center dot 12H(2)O. 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 P (1) over bar (No. 2). The crystal is a colorless block with the following parameters F.W. = 318.55, 0.54 x 0.24 x 0.18 mm(3), a) 3, 6.72548(7) angstrom, b = 6.77937(14) angstrom, c = 17.2898(5) angstrom, alpha = 88.255(1)degrees, beta = 89.478(2)degrees, gamma = 62.598(1)degrees, V = 699.54(3) angstrom(3) Z = 2, D-calc = 1.512 g/cm(3), mu = 0.343 mm(-1). Raman spectroscopy was used for characterizing MgSO4 center dot 11H(2)O and for comparing the vibrational spectra with the MgSO4 center dot 7H(2)O 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 center dot center dot center dot O (sulfate) and O-Mg-O bands vibrational modes. Thermogravimetric analysis confirmed the stochiometry of the MgSO4 center dot 11H(2)O salt. Additionally, the Miller indices of the major faces Of MgSO4 center dot 11H(2)O crystals were defined.