Redetermination and new description of the crystal structure of vanthoffite, Na6Mg(SO4)4

The crystal structure of vanthoffite {hexasodium magnesium tetrakis[sulfate(VI)]1, Na6Mg(SO4)(4), was solved in the year 1964 on a synthetic sample [Fischer & Hellner (1964). Acta Cryst. 17, 1613]. Here we report a redetermination of its crystal structure on a mineral sample with improved precision. It was refined in the space group P2i/c from a crystal originating from Surtsey, Iceland. The unique Mg (site symmetry 1) and the two S atoms are in usual, only slightly distorted octahedral and tetrahedral coordinations, respectively. The three independent Na atoms are in a distorted octahedral coordination (1 x) and distorted 7 -coordinations intermediate between a 'split octahedron' and a pentagonal bipyramid (2 x). [Mg06] coordination polyhedra interchange with one half of the sulfate tetrahedra in < 011 > chains forming a (100) meshed layer, with dimers formed by edge -sharing [Na07] polyhedra filling the interchain spaces. The other [Na07] polyhedra are organized in a parallel layer formed by [010] and [001] chains united through edge sharing and bonds to the remaining half of sulfate groups and to [Na06] octahedra. The two types of layers interconnect through tight bonding, which explains the lack of morphological characteristics typical of layered structures.