Crystals of a Vanthoffite mineral, Na6Mn(SO4)(4), grown from an aqueous solution, belong to a monoclinic system, P2(1)/c, Z = 2, at ambient temperature. Thermal analysis indicates a phase transition at 455 degrees C, which was substantiated by in situ variable-temperature powder X-ray diffraction. The structure is orthorhombic (Pmmm) after the phase transition and reverts to the monoclinic system upon cooling. Variable-temperature ionic conductivity measurements show a significantly higher value (similar to 10(-2) S cm(-1)) beyond the phase transition temperature.