Abstract
Members of the kentrolite-melanotekite series from L6ngban, Sweden, are orthorhombic holosymmetric, a = 6.961(2), b = 11.018(3), c: 9.964(5) Å, space group Pbcn (subgroup of Cmcm), Z = 4, formula from Cameca probe analysis and refined structure . The structure has been refined to R = 0.047 for 1031 reflections.
The fraction, the simple part, is based on edge-sharing octahedral chains parallel to [001]. These octahedra are alternately cis and trans with respect to adjacent octahedra, reminiscent of the structure of synthetic CMS-X1, . The oligosilicate dimers Si-O-Si are aligned nearly parallel to [00]. Their role is reminiscent of similar units in structures included in Belov's second chapter on silicates.
The 6s2Pb2+ lone-pair cations are split in an unexpected fashion, and this splitting was the source of the problems in structure solution. R ~ 0.14 typifies one Pb centroid alone. Gradual refinement of data from a ground sphere converged to 0.73 Pb(1) and 0.27 Pb(2), populated in a complementary fashion, with Pb(1) – Pb(2) separation of 0.56 Å. Noteworthy is the unsymmetrical splitting of the Pb atoms.
Bond distance averages are [6]M(1)3+-O = 2.01 (prolate spheroid), [6]M(2)3+-O = 2.03 (oblate spheroid), [4]Si-O = 1.63, [6]Pb(1)-O = 2.40–2.99, 〈2.60〉, and [6]Pb(2)-O = 2.27–3.02, 〈2.63〉 Å. Five cations and five anions comprise the asymmetric unit. Dominant 4d4 Mn3+ at M(1) and M(2) evince tetragonal Jahn-Teller distortion of the coordination poly-hedra as elongated (prolate) and compressed (oblate) octahedra, respectively.
The kentrolite-melanotekite structure type is another where cations show a close relationship to an intermetallic phase, in this case low URe2. For Pb(1), Pb(2), M(1), M(2), and Si cations and U, U, Re(1), Re(2), and Re(3) atoms, the mean difference between atom coordinates, scaled to the kentrolite cell, is Δ = 0.27 Å with range 0.00–0.48 Å.
