Experimental and Quasi-Experimental

atom completeness 0%; disorder in main residue; R factor = 0.029; wR factor = 0.058; data-to-parameter ratio = 19.1. 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. Related literature For the isotypic Co and Ni members of the M1-xM