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Lead-bearing phyllotungstite from the Clara mine, Germany with an ordered pyrochlore–hexagonal tungsten bronze intergrowth structure

  • I. E. Grey (a1), W. G. Mumme (a1) and C. M. MacRae (a1)

Abstract

Lead-bearing phyllotungstite from the Clara mine in the central Black Forest, Germany has a formula (Cs0.41)Na0.14K0.05Pb2+ 2.01Ca0.26[W6+ 10.87Fe3+ 3.13O35.75(OH)6.25](O(H2O)3). X-ray diffraction patterns exhibit pseudohexagonal symmetry, but refinement of single-crystal synchrotron data has shown that the true symmetry is orthorhombic, Cmcm, with a = 7.298(1), b = 12.640(2), c = 19.582(4) Å, and that the pseudohexagonal character is due to submicrometre-scale cyclical twinning by rotation about the pseudohexagonal c axis. The structure can be described in terms of an ordered intergrowth, parallel to (001), of (111)py blocks with pyrochlore-type structures, which are ~6 Å in width, and two-layer wide regions with a hexagonal tungsten bronze (HTB) type structure. Caesium atoms occupy 18-coordinate cavities in the HTB regions, and H2O molecules occupy Φ sites in the A 2 B 2O6Φ pyrochlore blocks. The lowering of symmetry from hexagonal to orthorhombic is due to partial ordering of W and Fe in the octahedral B sites and of Pb and vacancies in the A sites of the pyrochlore blocks. The ideal formula for the intergrowth structure (with no vacancies) is C 2A10[B14(O,OH)424, where C is the cavity site in the HTB slabs. The mineral has only 21% occupancy of the C site and 25% occupancy of the A site, but full occupancy of the Φ site. There may be some mixing of Cs and H2O between the C and Φ sites.

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Corresponding author

* E-mail: ian.grey@csiro.au

References

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Keywords

Type Description Title
UNKNOWN
Supplementary materials

Grey et al. supplementary material
CIF P63/mmc

 Unknown (45 KB)
45 KB
UNKNOWN
Supplementary materials

Grey et al. supplementary material
CIF Cmcm

 Unknown (94 KB)
94 KB

Lead-bearing phyllotungstite from the Clara mine, Germany with an ordered pyrochlore–hexagonal tungsten bronze intergrowth structure

  • I. E. Grey (a1), W. G. Mumme (a1) and C. M. MacRae (a1)

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