CRYSTAL CHEMISTRY OF THE ROSENBUSCHITE GROUP

Abstract

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 compo-sition, 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 Mn-dominated octahedra. Chemical data indicate that seidozerite does not form a solid solution with rosenbuschite.