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 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.

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 À 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.
The two metal ions are octahedrally surrounded by O atoms, with Mg-O distances in the range 2.018 (3) to 2.225 (3) Å. The Mg1O 6 octahedra share faces forming chains parallel to [001]. Mg2O 6 octahedra share faces and edges forming strands parallel to [001] (Fig. 1). Individual strands are linked with neighbouring strands through common corner atoms.
The AsO 3 (OH) tetrahedra flank the chains and strands and link both motifs into a three-dimensional framework (Fig. 2).
An interesting feature of this structure type is the short M···M contact within the chains of face-sharing M1O 6 octahedra running along [001]. The observed Mg···Mg distance of 2.5422 (1) Å corresponds to c/2 and lies between the respective distances of 2.5460 (1) Å for the Co and of 2.4843 (5) Å for the Ni member (Hughes et al., 2003).
The topological similarities between the framework structure of the title compound and that of the minerals dumortierite (Alexander et al., 1986) and cancrinite has been discussed in detail by Hughes et al. (2003). Experimental 200 mg of an amorphous precipitation product obtained by reacting MgCO 3 and arsenic acid (ca 20% wt ) was mixed with 300 mg HgO and placed in a Teflon container (volume 10 ml) that was filled up with two-thirds of its volume with water.
The inlay was placed in a steel autoclave and heated at 493 K for two weeks. Besides colourless needle-shaped crystals of the title compound, recrystallized HgO and α-(Hg 2 ) 3 (AsO 4 ) 2 (Weil, 2004b) were also present, as determined by singlecrystal X-ray diffraction of selected crystals.

Refinement
The atomic coordinates of the isotypic Co compound (Hughes et al., 2003) were used as starting parameters for refinement. The site occupation factor of Mg1 was refined freely; no significant deviation from full occupancy for Mg2 was observed. Hydrogen atoms could not be located reliably from difference maps and hence were not included in the refinement. According to the occupancy of Mg1, the overall number of H atoms was calculated as 4.5 for charge compensation. The maximum and minimum remaining electron densities were found 1.14 and 1.21 Å away from atom As2.