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

Original paper

Keller, Paul; Lissner, Falk; Schleid, Thomas

Abstract

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, I2/a, a = 11.835(2), ß = 6.328(1), c = 9.984(2) Å, ß = 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, ß = 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.

Keywords

stanekite • (fe3+ • mn2+ • fe2+ • mg)2[po4]o • crystal structure • fe2[po4]o • polymorphs • pegmatite • namibia