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

The crystal structure of vanthoffite {hexa­sodium magnesium tetra­kis[sulfate­(VI)]}, 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 P21/c from a crystal originating from Surtsey, Iceland. The unique Mg (site symmetry 1̅) 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×) and distorted 7-coordinations inter­mediate between a `split octa­hedron' and a penta­gonal bipyramid (2×). [MgO6] coordination polyhedra inter­change with one half of the sulfate tetra­hedra in <011> chains forming a (100) meshed layer, with dimers formed by edge-sharing [NaO7] polyhedra filling the inter­chain spaces. The other [NaO7] 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 [NaO6] octa­hedra. The two types of layers inter­connect through tight bonding, which explains the lack of morphological characteristics typical of layered structures. [ABSTRACT FROM AUTHOR]