Crystal structures of synthetic melanotekite (Pb2Fe2Si2O9), kentrolite (Pb2Mn2Si2O9), and the aluminum analogue (Pb2Al2Si2O9)

Synthetic crystals of melanotekite and kentrolite were obtained at 850 degrees C from melt. The aluminum analogue of kentrolite Pb2Al2Si2O9 was hydrothermally synthesized at 2 GPa, 650 degrees C together with zoisite-(Pb) and margarite-(Pb). Synthesis products were characterized by single-crystal diffraction studies and microprobe analysis. The aluminum analogue Pb2Al2O9 was observed in space group Pbcn with lattice parameters a = 6.8981(7) angstrom, b = 10.6906(15) angstrom, c = 9.7413(10) angstrom, and V= 718.37 angstrom(3). Fourier mappings show no irregularities of the Pb site. Melanotekite with lattice parameters a = 6.9786(6) angstrom, b = 11.0170(11) angstrom, c = 10.0895(9) angstrom, and V = 775.71(17) angstrom(3) in space group Pbcn show a slightly deformed Pb-position in Fourier mappings. Kentrolite was observed in space group P2(1)22(1) with pseudo-symmetry to Pbcn with lattice parameters a = 7.0103(5) angstrom, b = 11.0729 (7) angstrom, c = 9.9642(7) angstrom, and V= 773.47(11) angstrom(3). Fourier mappings of the kentrolite structure show that two different split Pb sites exist, which causes lower symmetry. The unit-cell volume of different members of the kentrolite group is a linear function of trivalent ionic radii in sixfold coordination for the elements Al, Ga, In, and also for Fe and Mn in high spin mode. The structure of Pb2M2Si2O9 (M = Al3+, Fe3+, Mn3+) is built on isolated M-octahedra chains parallel c, M-octahedra sharing alternately trans and skew edges. Each Si2O7-group is linked with their vertices to three octahedra chains. Their Si-O-Si bond angles depend on the size of M-octahedra and are 129.84 degrees in Pb2Al2Si2O9, 131.08 degrees in Pb2Fe2Si2O9, 128.34 degrees and 130.33 degrees in Pb2Mn2Si2O9.