Redetermination and new description of the crystal structure of vanthoffite, Na 6 Mg(SO 4 ) 4

• Published in: Acta Crystallographica Section E Crystallographic Communications
• Main Authors: Balić-Žunić, Tonči, Pamato, Martha G., Nestola, Fabrizio
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2020
• Subjects: Atomic coordinations; Crystal structure; Na-Mg sulfate; Redetermination; Vanthoffite; Iceland; Surtsey
• Online Access: https://curis.ku.dk/portal/da/publications/redetermination-and-new-description-of-the-crystal-structure-of-vanthoffite-na6mgso44(4ac31028-3a28-48fc-9f57-f10b351085b7).html; https://doi.org/10.1107/S2056989020005873; https://curis.ku.dk/ws/files/243377281/Redetermination_and_new_description_of_the_crystal_structure_of_vanthoffite_Na6Mg_SO4_4_.pdf

The crystal structure of vanthoffite {hexasodium magnesium tetrakis-[sulfate(VI)]}, Na 6 Mg(SO 4 ) 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 P2 1 /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×) and distorted 7-coordinations intermediate between a ‘split octahedron’ and a pentagonal bipyramid (2×). [MgO 6 ] coordination polyhedra interchange with one half of the sulfate tetrahedra in <011> chains forming a (100) meshed layer, with dimers formed by edge-sharing [NaO 7 ] polyhedra filling the interchain spaces. The other [NaO 7 ] 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 [NaO 6 ] octahedra. The two types of layers interconnect through tight bonding, which explains the lack of morphological characteristics typical of layered structures.