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The minerals of the rosenbuschite group are sorosilicates composed of a framework of 6-to 8-corner polyhedra and rows of Si 2 O 7 dimers. The polyhedra combine into layers (O layers) and into ribbons by edge sharing. Heterogeneous layers (H layers), composed of the octahedra from the ribbons and the sorosilicate groups, alternate with the O layer into a layered HOH structure. The 6-to 8-corner polyhedra host a variety of cations: Na, Mg, Ca, Ti, Mn, Fe, Y, Zr, Nb and the REE. Substitutions among these elements affect the geometrical properties of the various polyhedra. Crystal-structure refinements (X-ray diffraction) have been done on five specimens of the rosenbuschite group: götzenite, hainite, kochite (a new member of the group), rosenbuschite and seidozerite. Detailed models for their site occupancies are derived by fitting scattering values of the sites to the chemical composition, and the weighted bond-valence sums to valence sums in an integrated calculation procedure. Results of chemical analyses suggest a series of intermediate compositions between götzenite and kochite. This series may be described as a solid-solution series in which Zr substitutes for Ca in one structural position, götzenite being the Ca-rich end-member. Through substitution of Ti by Zr, still another solid-solution exists between kochite and rosenbuschite, with rosenbuschite as the Zr-rich member. The Ca → Zr substitution has significant effect on the size of the respective octahedron, as well as on the dimension and distortion of the adjacent polyhedra. The reduction in size of the Ca → Zr octahedron is partly compensated by an enlargement of the dimensions of the adjacent Ti octahedron. This change favors the Ti → Zr substitution at the latter site. Different degrees of distortion in the sorosilicate group and the adjacent Ca, Na octahedra are also associated with the Ca → Zr substitution. In the Zr-rich seidozerite, a stacking of the HOH structural layers different from the above-mentioned structures is observed. The change in stacking sequence is closely related to complex geometrical interrelationships between dimensions and distortions of the Zr-and Mndominated octahedra. Chemical data indicate that seidozerite does not form a solid solution with rosenbuschite.Keywords: rosenbuschite group, seidozerite, crystal chemistry, single-crystal X-ray diffraction, electron-microprobe data, cation order, polyhedron geometry. SOMMAIRELes minéraux du groupe de la rosenbuschite sont des sorosilicates dont la trame est composée de polyèdres à de six à huit coins et des rangées de dimères Si 2 O 7 . Les polyèdres sont agencés en couches (niveaux O) et en rubans par partage d'arêtes. Les couches hétérogènes (niveaux H), composée d'octaèdres des rubans et de groupes sorosilicatés, alternent avec les niveaux O pour former une structure stratifiée HOH. Les polyèdres à de six à huit coins renferment une variété de cations: Na, Mg, Ca, Ti, Mn, Fe, Y, Zr, Nb et les terres rares. Les substitutions impliquant ces éléments affectent les propri...

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The first comprehensive survey of band-edge features in the ternary group of naturally occurring aluminosilicates (the feldspars) is presented. Synchrotron-based luminescence excitation of KAlSi 3 O 8 , NaAlSi 3 O 8 and CaAl 2 Si 2 O 8 allows the measurement of the evolution of the band-gap across the system, which, at 8 K, is found to vary from 7.86 eV in NaAlSi 3 O 8 to 7.7 eV and 7.62 eV in KAlSi 3 O 8 and CaAl 2 Si 2 O 8 , respectively: the band-gap energies are typically 0.1 eV smaller at 300 K. In comparison with measurements made on natural and synthetic hydrothermal α-quartz, where both the direct and indirect band-gap structures are distinctly observable, no significant post-edge band structure is discernable in the feldspars. In Ca-rich material, the luminescence is attenuated by more than two orders of magnitude for excitation energies up to 3 eV above the band-gap, partly supporting the proposition (derived from previous cyclotron resonance experiments) that the bands are simply parabolic and that the materials may be direct band-gap insulators. The luminescence excitation experiments also allow an initial survey to be made of sub-band-gap features in the materials, including low mobility, temperature sensitive band-tail states and mid-gap defects.