Mg(6.75)(OH)(3)(H(0.166)AsO(4))(3)(HAsO(4)), a member of the M (1-) (x)M′(6)(OH)(3)(H(2x/3)AsO(4))(3)(HAsO(4)) family (M,M′ = Co; Ni)

In the structure of the title compound, magnesium hydroxide hydrogenarsenate (6.75/3/4), two different Mg(2+) ions, one located on a site with symmetry 3m. (occupancy 3/4) and one on a general position, as well as two different AsO(3)(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 Mg(2+) ions are octa­hedrally surrounded by O atoms. The MgO(6) octa­hedra belonging to the partially occupied Mg(2+) sites share faces, forming chains along [001]. The other type of MgO(6) octa­hedra share corners and faces under formation of strands parallel to [001] whereby individual strands are linked through common corner atoms. The two types of AsO(3)(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.