CRYSTAL-CHEMISTRY AND OPTICS OF BAZZITE FROM FURKABASIS TUNNEL (SWITZERLAND)

The crystal structure of blue bazzite (space group P6/mcc, Z = 2, a = 9.501(1), c = 9.178(1) angstrom) with the composition Be3(Sc1.25Fe3+ 0.43Mg0.32Mn0.03Al0.02)SIGMA = 2.05.Si6O18[Na0.32(n H2O)] was refined from X-ray single-crystal data with 422 unique reflections to R = 2.2%. The structure is of the beryl type with octahedra strongly compressed parallel to the c-axis. The octahedral Me-O distance in bazzite is 2.080 angstrom compared to 1.904 angstrom in beryl. The flattening of octahdra leads to a larger a cell dimension in bazzite compared to beryl (9.209 angstrom). Two-valent cations (mainly Mg) in octahedral coordination are charge balanced by Na at (0,0,0) in the structural channels. Polarized single-crystal IR-spectra recorded between 400 and 8000 cm-1 indicate that H2O in the structural channels is oriented with the H-H vector perpendicular to the c-axis. The IR-spectra show more absorption bands than known for type II H2O in beryl thus the existence of more than one H2O species or even OH-groups is very likely. Refractive indices of the same bazzite were measured using a spindle-stage and employing the wavelength-temperature variation method yielding n(o) = 1.6279(3), n(e) = 1.6066(5) for 589 nm at 25-degrees-C. The birefringence DELTA = 0.0213 is significantly larger than the one of near end-member beryl (DELTA = 0.0047). This high birefringence of bazzite is related to the electronic polarizability of octahedral Sc and Fe which increase n(o) at a stronger rate than n(e). Transition metals in bazzite are also responsible for a higher refractive index dispersion than found for beryl.