Mg6.75(OH)3(H0.166AsO4)3(HAsO4), a member of the M 1- xM'6(OH)3(H2x/3AsO4)3(HAsO4) 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 AsO3(OH) tetra-hedra (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 octa-hedrally surrounded by O atoms. The MgO6 octa-hedra belonging to the partially occupied Mg(2+) sites share faces, forming chains along [001]. The other type of MgO6 octa-hedra 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) tetra-hedra inter-link 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.