Received 18 April 2013
In the structure of the title compound, magnesium hydroxide hydrogenarsenate (6.75/3/4), two different Mg2+ ions, one located on a site with symmetry 3m. (occupancy 3/4) and one on a general position, as well as two different AsO3(OH) tetrahedra (symmetry .m. with partial occupancy for the H atom for one, and symmetry 3m. with full occupancy for the H atom for the other) and one OH- ion (site symmetry .m.) are present. Both Mg2+ ions are octahedrally surrounded by O atoms. The MgO6 octahedra belonging to the partially occupied Mg2+ sites share faces, forming chains along . The other type of MgO6 octahedra share corners and faces under formation of strands parallel to  whereby individual strands are linked through common corner atoms. The two types of AsO3(OH) tetrahedra interlink the strands and the chains, building up a three-dimensional framework resembling that of the mineral dumortierite. The OH groups were assigned on basis of bond-valence calculations and crystal chemical considerations.
For the isotypic Co and Ni members of the M1-xM'6(OH)3(H2x/3AsO4)3(HAsO4) series, see: Hughes et al. (2003). For other reaction products obtained under the given or similar hydrothermal conditions, see: Weil (2004a,b). For the crystal structure of dumortierite, see: Alexander et al. (1986). The bond-valence method has been described by Brown (2002).
Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: VN2069 ).
The X-ray centre of the Vienna University of Technology is acknowledged for financial support and for providing access to the single-crystal diffractometer.
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