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

¹ Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
² Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany

^* Corresponding author, e-mail: paul.keller{at}mineralogie.uni-stuttgart.de

Two monoclinic polytypes of stankite, (Fe³⁺,Mn²⁺,Fe²⁺,Mg)₂[PO₄]O, were found at the type locality Okatjimukuju, Karibib (Namibia). The crystal structure of stankite-Mabc, 12/a, a = 11.835(2), b = 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, stankite-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. Stankite, synthetic - and ß-Fe₂[PO₄]O are polymorphs. In contrast to stankite, the synthetic polymorphs are characterized by face-sharing [MO₆] octahedra, which join together to straight or tightly folded zigzag chains for both ß-Fe₂[PO₄]O-I and ß-Fe₂[PO₄]O-II or -Fe₂[PO₄]O, respectively.

Key-words: stankite, (Fe³⁺,Mn²⁺,Fe²⁺,Mg)₂[PO₄]O, crystal structure, Fe₂[PO₄]O, polymorphs, pegmatite, Namibia.

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