Lead-bearing phyllotungstite from the Clara mine in the central Black Forest, Germany has a formula (Cs-0.41)Na0.14K0.05Pb2.012+Ca0.26[W10.876+Fe3.133+O35.75(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) angstrom, 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 similar to 6 angstrom 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 Phi sites in the A(2)B(2)O(6)Phi 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(2)A(10)[B-14(O,OH)(42)]Phi(4), 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 Phi site. There may be some mixing of Cs and H2O between the C and Phi sites.
