The Al-Si configuration in ordered and disordered lead feldspar (PbAl2Si2O8) has been investigated by single-crystal X-ray diffraction. Single crystals synthesized from melt and cooled from T = 1280 to 1000 degrees 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 reflections (C2/m, a = 8.428, b = 13.054, c = 7.174 Angstrom, beta = 115.32 degrees; R = 5.8%). Subsequent structure refinement of a crystal hydrothermally annealed at T = 500 degrees C and P-H2O = 2 kbar for 216 h shows 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 the ordered lead feldspar the Pb polyhedron is significantly distorted in comparison with the Sr polyhedron in strontium feldspar. The coordination of the Pb site is reduced from sevenfold, as in strontium feldspar, to a rather irregular sixfold configuration. Such distortion could be related to the lone-pair effect in Pb2+. In the disordered lead feldspar the Fourier map shows an anomalous electron-density distribution around the Pb site and consequently a split-site model for Pb was assumed in the refinement (Pb-Pb' split = 0.557 Angstrom).
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