The crystal structure of staněkite, (Fe3+, Mn2+,Fe2+,Mg)2[PO4]O, from Okatjimukuju, Karibib (Namibia), and its relationship to the polymorphs of synthetic Fe2 Fe2[PO4]O

Two monoclinic polytypes of staněkite, (Fe³⁺,Mn²⁺,Fe²⁺,Mg)₂[PO₄]O, were found at the type locality Okatjimukuju, Karibib (Namibia). The crystal structure of staněkite-Mabc, 12/a, a = 11.835(2), b = 6.328(1), c = 9.984(2) Å,β = 105.81(3)° Z = 8, was determined from single-crystal X-ray intensity data and refined up to R₁ = 0.0385. The other polytype, staněkite-Ma2bc, P2₁/a, a = 11.844(3), b = 12.663(3), c = 9.989(3) Å,β = 105.93(2) °, Z = 16, is characterized by weak additional reflections and only the substructure could be refined to an acceptable R₁ = 0.0450. Only small structural differences exist between both polytypes. The crystal structure consists of a rather regular [PO₄] tetrahedron and two symmetrically different strongly distorted [MO₆] octahedra. Independent of their valency, the cations M are distributed over the octahedral sites. The [MO₆] octahedra share common edges to form zigzag chains running parallel to [010] for [(M1)O₆] and to [100] for [(M2)O₆], respectively. Both chain types are further interconnected by additional edges along the c axis to build an open framework that hosts the [PO₄] tetrahedra. Staněkite, synthetic α- and β-Fe₂[PO₄]O are polymorphs. In contrast to staněkite, the synthetic polymorphs are characterized by face-sharing [MO₆] octahedra, which join together to straight or tightly folded zigzag chains for both β-Fe₂[PO₄]O-I and β-Fe₂[PO₄]O-II or α-Fe₂[PO₄]O, respectively.