Magnussonite, manganese arsenite, a fluorite derivative structure

A single crystal of magnussonite, nd-member formula unit Mn?r+ [As[+ Mnl+O,r]rClr, was studied in detail by three-dimensional X-ray diffractometry. The mineral is cubic, space group Ia3d,Z:8, a: 19.680(4)4. R:0.112(R, = 0.097) for 1373 independentref lect ions.X-ray diffraction data to sindltr: 0.80 (MoKa 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 fluorite-l ike formula can be written XrrOruErr, Z: Swhere! areordered vacanciesovertheanion frame. Its underlying principle is a large cluster of composition [As3+Mnr+Or.], where the oxygens define a polyhedron of point symmetry 3 consisting of 36 edges, l8 vertices, 2 hexagonal faces, l2 triangular faces, and 6 quadrilateral faces. The arsenite oxygens reside on the periphery, and the central core consists of six Ass+octahedrally coordinated to central Mn1+ with an average bond distance Ass+-Mnt+: 2.65A,0.084 longer than Mn-As-- 2.57 A found in the structure of MnAs (nickel arsenide structure type). This unusual structure is interpreted as satisfying the 18-electron rule with 2x 6-- 12 electrons donated by theAss+ plus 6 d electrons from Mnl+. The Cl- ions reside outside the large hexagonal faces of the oxygen polyhedron. The rest of the structure consists of Mn(1)O, distorted cubes, Mn(2)O, distorted squares, Mn(3)Ou distorted trigonal prisms, and Mn(4)O. distorted octahedra