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

¹ Fachgebiet Mineralogie und Petrologie, Technischen Universität Berlin Ackerstr. 76, D-13355 Berlin, Germany
² GeoForschungsZentrum Potsdam, Department 4, Telegrafenberg, D-14473 Potsdam, Germany

Correspondence: ^* E-mail: guido.doersam{at}web.de

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 ų. 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) ų 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) ų. 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.

Key Words: Kentrolite • melanotekite • Pb₂Al₂Si₂O₉ • Pb₂Fe₂Si₂O₉ • Pb₂Mn₂Si₂O₉ • crystal-structure • X-ray-diffraction • EMP-analysis