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

摘要

Two monoclinic polytypes of stanekite, (Fe3+, Mn2+, Fe2+ Mg)(2)[PO4]O, were found at the type locality Okatjimukuju, Karibib (Namibia). The crystal structure of stanekite-Mabc, I2/a, a = 11.835(2), b = 6.328(1), c = 9.984(2) angstrom, beta 3 = 105.81(3)degrees, Z = 8, was determined from single-crystal X-ray intensity data and refined up to R-1 = 0.0385. The other polytype, stanekite-Ma2bc, P2(1)/a, a = 11.844(3), b = 12.663(3), c = 9. 989(3) angstrom, beta = 105.93(2)degrees, Z = 16, is characterized by weak additional reflections and only the substructure could be refined to an acceptable R-1 = 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)O-6] and to [100] for [(M-2)O-6], respectively. Both chain types are further interconnected by additional edges along the c axis to build an open framework that hosts the [PO4] tetrahedra. Stanekite, synthetic alpha- and beta-Fe-2[PO4]O are polymorphs. In contrast to stanekite, the synthetic polymorphs are characterized by face-sharing [MO6] octahedra, which join together to straight or tightly folded zigzag chains for both beta-Fe-2[PO4]O-I and beta-Fe-2[PO4]O-II or alpha-Fe2 [PO4]O, respectively.