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

Lead-bearing phyllotungstite from the Clara mine in the central Black Forest, Germany has a formula (Cs0.41)Na0.14K0.05Pb2+2.01Ca 0.26[W6+10.87Fe3+3.13O 35.75(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 caxis. The structure can be described in terms of an ordered intergrowth, parallel to (001), of (111)pyblocks 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 F sites in the A2B2O6F pyrochlore blocks. The lowering of symmetry from hexagonal to orthorhombic is due to partial ordering of W and Fe in the octahedral Bsites and of Pb and vacancies in the Asites of the pyrochlore blocks. The ideal formula for the intergrowth structure (with no vacancies) is C2A 10[B14(O, OH)42] F4, where Cis the cavity site in the HTB slabs. The mineral has only 21% occupancy of the Csite and 25% occupancy of the Asite, but full occupancy of the F site. There may be some mixing of Cs and H2O between the Cand F sites. [ABSTRACT FROM AUTHOR]