Publication Date:
2013-02-21
Description:
Lead-bearing phyllotungstite from the Clara mine in the central Black Forest, Germany has a formula (Cs 0.41 )Na 0.14 K 0.05 $${\mathrm{Pb}}_{2.01}^{2+}$$ Ca 0.26 [ $${\mathrm{W}}_{10.87}^{6+}$$ $${\mathrm{Fe}}_{3.13}^{3+}$$ O 35.75 (OH) 6.25 ](O(H 2 O) 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 H 2 O molecules occupy sites in the A 2 B 2 O 6 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 ] 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 site. There may be some mixing of Cs and H 2 O between the C and sites.
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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