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The crystal structures of kidwellite and ‘laubmannite’, two complex fibrous iron phosphates

Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstrasse 14, A-1090 Wien, Austria

^* E-mail: uwe.kolitsch{at}univie.ac.at

The previously unknown, complex crystal structures of two fibrous ferric iron phosphate minerals have been solved using single-crystal X-ray diffraction data. The structure of a slightly arsenatian kidwellite has been refined in space group P2/c (a = 20.117(4), b = 5.185(1), c = 13.978(3) Å, ß = 107.07(3)°, V = 1393.8(5) Å ³, Z = 2) to R1 = 5.21%; a revision of both space group symmetry and chemical formula is proposed. The idealized formula is Na(Fe³⁺,M)_9+x(OH)11(H₂O)₃(PO₄)₆, where M = Fe³⁺, Cu²⁺ or other metal cation, and x 0.3. The structure of a slightly arsenatian ‘laubmannite’ (as defined by Moore, 1970) has been refined in space group Pbcm (a = 5.172(1), b = 13.999(3), c = 31.083(6) Å, V = 2250.5(8) Å ³, Z = 4) to R1 = 3.14%. The revised, idealized formula is (Fe³⁺,Fe²⁺,M)_8+x(OH,H₂O)₉(H₂O)₂(PO₄)₅, where M = Fe³⁺, Cu²⁺ or other metal cation, and x 0.1. The framework structures of both minerals are similar. Dominant building units are dimers composed of face- and edge-sharing FeO₆ octahedra. Whereas kidwellite contains an additional trimer built of three corner-sharing FeO₆ octahedra, ‘laubmannite’ instead contains a dimer built of two corner-sharing FeO₆ octahedra. Kidwellite contains only trivalent iron, while one of the Fe sites in ‘laubmannite’ is occupied by a mixture of Fe³⁺ and Fe²⁺ in a 1:1 ratio. In both structures, the FeO₆-based building units are linked via corners to PO₄ tetrahedra; the M sites are located in narrow channels and have very low occupancies (~2 to 7%) and strongly distorted [6]- or [5+1]-coordinations. Close structural relations between kidwellite and ‘laubmannite’, and other fibrous iron phosphates explain observations of epitaxial intergrowths of them.

KEYWORDS: kidwellite, laubmannite, crystal structure, crystal chemistry, fibrous iron phosphates

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