The Mg member of the isotypic series MTe₆O₁₃

MgTe₆O₁₃, magnesium hexa­tellurate(IV), is isotypic with the structures of divalent first-row transition metal analogues MTe₆O₁₃ (M = Mn, Fe, Co, Ni and Zn). The asymmetric unit contains one Mg, two Te and five O atoms of which the Mg and one O atom lie on a threefold rotation axis. The structure is made up from slightly distorted [MgO₆] octa­hedra (isolated from each other), distorted [TeO₄] bis­phenoids and [TeO_4 + 1] tetra­gonal pyramids sharing corners and edges. This arrangement leads to the formation of a dense three-dimensional structure.

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.

Redetermination of MgCrO₄·5H₂O

The CCD-data based redetermination of the crystal structure of the title compound, magnesium chromate(VI) penta­hydrate, confirms in principle the previous study based on precession film data [Bertrand et al. (1971). C. R. Hebd. Seances Acad. Sci. Serie C, 272, 530–533.], but with all atoms refined with anisotropic displacement parameters and with all H atoms localized. This allowed an unambiguous assignment of the hydrogen-bonding pattern. MgCrO₄·5H₂O adopts the MgSO₄·5H₂O structure type. It contains two Mg²⁺ sites on special positions with site symmetry -1, one tetra­hedral CrO₄ group and five water mol­ecules. Four of them coordinate to the Mg²⁺ cation, and one is an uncoordinating lattice water mol­ecule. The octa­hedral environment of the Mg²⁺ cation is completed by two axial O atoms of CrO₄ tetra­hedra. This arrangement leads to the formation of chains parallel to [011]. Adjacent chains are linked through O—H...O hydrogen bonds (one of them bifurcated), involving both the coordi­nating and lattice water mol­ecules, into a three-dimensional network.

Olivine-type NaCd(AsO₄)

The title compound, sodium cadmium orthoarsenate, adopts the olivine [Mg₂(SiO₄)] structure type in space group Pnma, with Na (site symmetry -1) and Cd (.m.) replacing the two Mg positions, and the AsO₄ tetra­hedron (.m.) the SiO₄ tetra­hedron. The crystal structure is made up of a nearly hexa­gonal closed-packed arrangement of O atoms stacked along [001]. The Na and Cd atoms occupy one half of the octa­hedral voids in alternate layers stacked along [100], and one eighth of the tetra­hedral voids are occupied by As atoms.