The crystal structures of kidwellite and 'laubmannite', two complex fibrous iron phosphates
- Author(s)
- Uwe Kolitsch
- Abstract
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(l), c - 13.978(3) A°, ß = 107.07(3)°, V = 1393.8(5) A°3, Z = 2) to RI = 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(l), b = 13.999(3), c = 31.083(6) A°, V = 2250.5(8) A°3, Z = 4) to RI = 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. Œ 2004 The Mineralogical Society.
- Organisation(s)
- Department of Mineralogy and Crystallography
- Journal
- Mineralogical Magazine
- Volume
- 68
- Pages
- 147-165
- No. of pages
- 19
- ISSN
- 0026-461X
- Publication date
- 2004
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 1051 Geology, Mineralogy
- Portal url
- https://ucris.univie.ac.at/portal/en/publications/the-crystal-structures-of-kidwellite-and-laubmannite-two-complex-fibrous-iron-phosphates(ac4e5d4d-a52d-4296-a849-617998c35be3).html