NaSr(AsO(4))(H(2)O)(9): the (Sr,As) analogue of nabaphite and nastrophite.

The crystal structure of the title compound, sodium strontium orthoarsenate(V) nona-hydrate, is isotypic with NaSr(PO(4))(H(2)O)(9) and the minerals nabaphite [NaBa(PO(4))(H(2)O)(9)] and nastrophite [Na(Sr,Ba)(PO(4))(H(2)O)(9)]. The Na and Sr atoms are located on threefold rotation axes and are in the centres of slightly distorted Na(H(2)O)(6) octa-hedra and Sr(H(2)O)(9) tricapped trigonal prisms, respectively. A framework structure is established via edge-sharing of these polyhedra. Disordered AsO(4) tetra-hedra (with threefold symmetry) are situated in the inter-stitial space of the framework. Although reasonable H-atom positions of the water mol-ecules were not established, close O⋯O contacts between the disordered AsO(4) tetra-hedra and the water mol-ecules suggest strong O-H⋯O hydrogen bonding.