Synthesis of Pb-zoisite and Pb-lawsonite

摘要

Hydrothermal syntheses of Pb-zoisite Pb_2Al_3[SiO_4|Si_2O_7|O|OH] and Pb-lawsonite PbAl_2[Si_2O_7|(OH)_2]·H_2O were performed at high pressure and temperature conditions with standard piston cylinder press experiments. Starting materials were mixtures of PbAl_2O_4, SiO_2, PbO and H_2O. The run products were characterized by single-crystal and powder X-ray diffraction, scanning electron microscopy and electron microprobe analyses. Idiomorphic colourless Pb-zoisite crystals with sizes of 60 × 50 × 120 μm were obtained at 2 GPa and 600 ℃, together with Pblawsonite and traces of Pb-margarite and plumbotsumite Pb_5Si_4O_8(OH)_(10). Single-crystal diffraction studies and structure solution of Pb-zoisite yielded space group Pnma (62), Z = 4, a = 16.4529(7) A, b = 5.6432(2) A, c = 10.3631(5) A, V = 962.18 A~3, R1 = 0.067. Pb-lawsonite was obtained at 3 GPa/600 ℃ and at 2 GPa/400 ℃. Powder-XRD pattern of Pb-lawsonite shows an orthorhombic unit cell. Peaks at (101), (103), (121), (211), (212), (213), (231), (301), (233) suggest space group Pbnm (62) with Z = 4, a = c 5.85 A, b = 9.03 A, c = 13.31 A, V = 703 A~3, instead of space group Cmcn for Ca-lawsonite and P2_1/m for Sr-lawsonite. Group-subgroup relations of the lawsonite structure family are presented.