Redetermination of the crystal structure of löweite, Na12Mg7(SO4)13(H2O)15

Abstract The crystal structure of löweite, Na₁₂Mg₇(SO₄)₁₃(H₂O)₁₅, has been redetermined using single-crystal X-ray diffraction on a synthetic crystal to resolve ambiguities present in the original structure reported by Fang and Robinson (1970). The new data were collected with a modern charge-coupled device (CCD)-equipped diffractometer and refined anisotropically. The resulting structure has a trigonal unit cell [space group R $$\:\overline3$$ , a = 18.8483(5) Å, c = 13.4190(4) Å, Z = 3, R 1 = 0.0269, R w =0.0345]. It confirms previous results while revealing new details, including anisotropically refined oxygen positions with resolved overlaps and most hydrogen positions. Hydrogen atom positions were determined for the two symmetry-independent water molecules’ sites, while they remain unclear for the third one. This redetermination provides a more accurate model of the coordination environments of Mg and Na atoms. It clarifies the features of the hydration and bonding in löweite, with a more detailed description of its crystal structure. It is built on a three-dimensional framework with the composition Na 12 Mg 6 (SO 4 ) 12 (H 2 O) 12 in which hydrogen bonding plays a minor role. This part of the structure conforms with the general chemistry of the Na-Mg sulfate hydrate series and explains the high thermal stability of löweite. The framework contains channels where additional Mg atoms, sulphate groups, and water molecules are situated. This part of the structure contributes to an unusually low Na:Mg ratio, compared to other hydrated Na-Mg sulfates. The channel water molecules exhibit large, anharmonic displacements, possibly due to a combination of bonding to framework Na atoms and hydrogen bonding to an oxygen atom of the channel sulphate group.