Magnussonite, manganese arsenite, a fluorite derivative structure Available to Purchase

A single crystal of magnussonite, end-member formula unit $Mn182+$[$As63+$$Mn181+$O]₂Cl₂, was studied in detail by three-dimensional X-ray diffractometry. The mineral is cubic, space group laid, Z = 8, a = 19.680(4)A. R = 0.112 (R_w = 0.097) for 1373 independent reflections. X-ray diffraction data to sinθ/λ = 0.80 (MoKα radiation) were collected on a Pailred diffractometer, and the structure was solved by Patterson, Fourier, and least-squares refinement techniques.

The structure is an anion-deficient derivative of the fluorite structure type and possesses a pronounced substructure a′ = a/4. The cell contains 64 fluorite cells and the general fluoritelike formula can be written X₃₂O₃₆□₂₈ Z = 8 where □ are ordered vacancies over the anion frame. Its underlying principle is a large cluster of composition [$As63+$Mn¹⁺O₁₈], where the oxygens define a polyhedron of point symmetry 3 consisting of 36 edges, 18 vertices, 2 hexagonal faces, 12 triangular faces, and 6 quadrilateral faces. The arsenite oxygens reside on the periphery, and the central core consists of six As³⁺octahedrally coordinated to central Mn¹⁺ with an average bond distance As³⁺−Mn¹⁺ = 2.65A, 0.08A longer than Mn−As = 2.57A found in the structure of MnAs (nickel arsenide structure type). This unusual structure is interpreted as satisfying the 18-electron rule with 2×6 = 12 electrons donated by the As³⁺ plus 6 d⁸ electrons from Mn¹⁺. The Cl⁻ ions reside outside the large hexagonal faces of the oxygen polyhedron.

The rest of the structure consists of Mn(1)O₈ distorted cubes, Mn(2)O₄ distorted squares, Mn(3)O₆ distorted trigonal prisms, and Mn(4)O₆ distorted octahedra.