Synthetic crystals of melanotekite and kentrolite were obtained at 850 °C from melt. The aluminum analogue of kentrolite Pb2Al2Si2O9 was hydrothermally synthesized at 2 GPa, 650 °C together with zoisite-(Pb) and margarite-(Pb). Synthesis products were characterized by single-crystal diffraction studies and microprobe analysis.
The aluminum analogue Pb2Al2Si2O9 was observed in space group Pbcn with lattice parameters a = 6.8981(7) Å, b = 10.6906(15) Å, c = 9.7413(10) Å, and V = 718.37 Å3. Fourier mappings show no irregularities of the Pb site.
Melanotekite with lattice parameters a = 6.9786(6) Å, b = 11.0170(11) Å, c = 10.0895(9) Å, and V = 775.71(17) Å3 in space group Pbcn show a slightly deformed Pb-position in Fourier mappings.
Kentrolite was observed in space group P21221 with pseudo-symmetry to Pbcn with lattice parameters a = 7.0103(5) Å, b = 11.0729 (7) Å, c = 9.9642(7) Å, and V = 773.47(11) Å3. 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° in Pb2Al2Si2O9, 131.08° in Pb2Fe2Si2O9, 128.34° and 130.33° in Pb2Mn2Si2O9.