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The Al-Si configuration in ordered and disordered lead feldspar (PbAl 2 Si 2 O 8 ) has been investigated by single-crystal X-ray diffraction. Single crystals synthesized from melt and cooled from T = 1280 to 1000 °C in 1 h and then to T= 700 °C in 15 h show a completely disordered Al-Si configuration with no b&gt;-type superstructure reflections (C2/m, a = 8.428, b = 13.054, c = 7.174 Å, p = 115.32°; R = 5.8%). Subsequent structure refinement of a crystal hydrothermally annealed at T = 500 °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 Å , p = 115.19°; R = 4.7%; Q od  ≈ 0.9; 950 b reflections with F o  ≥ 4σ). 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 Pb 2+ . 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 Å).

The structure of ordered and disordered lead feldspar (PbAl 2 Si 2 O 8 )