Disodium nickel bis(sulfate) tetrahydrate: a nickel astrakanite

The title compound, [Na2Ni(SO4)(2)(H2O)(4)] or Ni astrakanite, is part of an isostructural family containing the Mg (the original astrakanite mineral), Zn and Co species. The very regular NiO(H2O)(4)O(SO4)(2) octahedra lie at centers of symmetry, while the rather distorted NaO(H2O)(2)O(SO4)(4) octahedra are at general positions, linked into a three-dimensional network by the bridging water molecules and fully coordinated sulfate groups.