Fillowite, Na2Ca(Mn,Fe)72+(PO4)6⁠: its crystal structure

Fillowite, $Na2Ca(Mn,Fe)72+(PO4)6$⁠, possesses a structure derived from the glaserite, K₃Na(SO₄)₂, arrangement. No evidence of substructure occurs either in the X-ray data or the polyhedral arrangement. It is rhombohedral, a₁ 15.282(2), c 43.507(3)Å, Z = 18, space group R3. R = 0.069 for 6891 independent reflections. Forty-five nonequivalent atoms occur in the asymmetric unit. Nine distinct MO₆ octahedra, four distinct five-coordinate polyhedra, one seven-coordinate polyhedron, one eight-coordinate polyhedron and six distinct PO₄ tetrahedra occur in the asymmetric unit. The eight-coordinate polyhedron, maximal point symmetry 4m2, is the gable disphenoid and is identical to a polyhedron in the related mineral wyllieite, $Na2Fe22+Al(PO4)3$⁠. In these structures, the ratio of larger cations to tetrahedra is X:T = 5:3.

Three kinds of rods occur in the structure and, when projected down the c-axis, define the regular {6³} hexagonal tessellation. Rod I at the origin has the cation sequence ⋯ M(1)–M(3)–Na(1)–Na(2)–M(4)–M(5)–M(2)–M(5)–M(4)–Na(2)–Na(1)–M(3) ⋯ with five octahedra in sequence sharing faces: M(3)–Na(1)–Na(2)–M(4)–M(5). Rod II along the 3₁-axis has ⋯ Ca–M(9)–Na(3)–M(8)–Ca–M(9)–Na(3)–M(8)–Ca–M(9)–Na(3)–M(8) ⋯ Rod III at the nodes of the hexagonal net includes the six nonequivalent PO₄ tetrahedra, ⋯ P(6)–M(11)–P(4)–M(6)–P(1)–☐(1)–P(5)–M(7)–P(2)–M(10)–P(3)–☐(2) ⋯. Counting up the cations, the ratio of larger cations (X) to tetrahedra (T) is ☐₈X₄₀T₂₄ or X:T = 5:3.