THE STRUCTURE OF ORDERED AND DISORDERED LEAD FELDSPAR (PBAL2SI2O8)

The Al-Si configuration in ordered and disordered lead feldspar (PbAl2 Si2O8) has been investigated by single-crystal X-ray diffraction. Sing le crystals synthesized from melt and cooled from T = 1280 to 1000 deg rees C in 1 h and then to T = 700 degrees C in 15 h show a completely disordered Al-Si configuration with no b-type superstructure reflectio ns (C2/m, a = 8.428, b = 13.054, c = 7.174 Angstrom, beta = 115.32 deg rees; R = 5.8%). Subsequent structure refinement of a crystal hydrothe rmally annealed at T = 500 degrees C and P-H2O = 2 kbar for 216 h show s an ordered Al-Si configuration (I2/c, a = 8.388, b = 13.067, c = 14. 327 Angstrom, beta = 115.19 degrees; R = 4.7%; Q(od) approximate to 0. 9; 950 b reflections with F-o greater than or equal to 4 sigma). In th e ordered lead feldspar the Pb polyhedron is significantly distorted i n comparison with the Sr polyhedron in strontium feldspar. The coordin ation of the Pb site is reduced from sevenfold, as in strontium feldsp ar, to a rather irregular sixfold configuration. Such distortion could be related to the lone-pair effect in Pb2+. In the disordered lead fe ldspar the Fourier map shows an anomalous electron-density distributio n around the Pb site and consequently a split-site model for Pb was as sumed in the refinement (Pb-Pb' split = 0.557 Angstrom).