Mg_6.75(OH)₃(H_0.166AsO₄)₃(HAsO₄), a member of the M_1-xM'₆(OH)₃(H_2x/3AsO₄)₃(HAsO₄) family (M,M' = Co; Ni)

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 [001]. The other type of MgO₆ 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 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.