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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

View larger version (33K): [in this window] [in a new window]   FIGURE 1. Crystal structure of Pb2Al2Si2O9 in space group Pbcn, (a) projection from a on the b–c plane, and (b) from c on the a–b plane.

View larger version (64K): [in this window] [in a new window]   FIGURE 2. BSE-image shows euhedral kentrolite crystals with small braunite inclusions in a matrix with Pb2SiO4 composition.

View larger version (18K): [in this window] [in a new window]   FIGURE 3. Fourier-mappings (Fobs) of the Pb sites in kentrolite-(Al), melanotekite, and kentrolite illustrating differences in electron density (16 contour levels with 20 e/Å3).

View larger version (15K): [in this window] [in a new window]   FIGURE 4. Changes in lattice parameters as a function of ionic radii (Shannon 1976) of trivalent cations (Fe, Mn in high spin) on the M-positions in sixfold coordination in kentrolite isotypes. Open squares represent data for melanotekite and kentrolite (this study); filled squares represent the kentrolite isotypes of Al, In, and Ga (Al = this study; In = Werner and Müller-Buschbaum 1997; Ga = Gabelica-Robert and Tarte 1979) open triangles represent Cr and Sc-isotypes (Gabelica-Robert and Tarte 1979).

View larger version (8K): [in this window] [in a new window]   FIGURE 5. Fourier maps (Fobs) with projection from c on the a–b plane, Pb-positions are shown for kentrolite-(Al) in Pbcn (a), melanotekite in Pbcn (b) and kentrolite in P21221 (c).

View larger version (31K): [in this window] [in a new window]   FIGURE 6. Crystal structure of (a) kentrolite-(Al), (b) melanotekite, and (c) kentrolite, projected from c on the a–b plane. Lead positions with high site occupation factor (SOF) are shown as black and gray filled circles. Split lead positions with low SOF are shown as small white or gray filled circles.

View larger version (28K): [in this window] [in a new window]   FIGURE 7. Differences in octahedral chain in (a) kentrolite-(Al) compared to (b) kentrolite-(In). Octahedra chain in kentrolite-(Al) is stretched. Black dots represent Al1 and Al2 atoms in kentrolite-(Al), respectively, In1 and In2 atoms in Kentrolite-(In). Differences in M-octahedra of (c) melanotekite and (d) kentrolite is shown with bond-length. The shortest bonds of each octahedon are in gray.

View larger version (14K): [in this window] [in a new window]   FIGURE 8. Comparison of mean M-O distances of M3+ (M = Al, Ga, In, Tl) octahedra in Pb2M2Si2O9 and M2O3 as a function of ionic radii (Shannon 1976).

View larger version (34K): [in this window] [in a new window]   FIGURE 9. Projection of the melanotekite structure from a on the b–c plane. (PbFe)O-cluster with O5 as central atom builds a 3D-network. The Si2O7-group is placed in cavities of the cluster-network. Additional Fe-O and Pb-O bonds are shown as solid black and white lines, respectively.

View larger version (16K): [in this window] [in a new window]   FIGURE 10. [PbFeO]-cluster displacement by split Pb site.