Crystal chemistry and optics of bazzite from Furkabasistunnel (Switzerland)

The crystal structure of blue bazzite (space group P6/mcc, Z = 2, α = 9.501(1), c = 9.178(1) Å) with the composition Be₃(Sc_1.25Fe³⁺_0.43Mg_0.32Mn_0.03Al_0.02)Σ_=2.05 Si₆O₁₈[Na_0.32(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 A compared to 1.904 Å in beryl. The flattening of octahedra leads to a larger a cell dimension in bazzite compared to beryl (9.209 A). 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°C. The birefringence Δ = 0.0213 is significantly larger than the one of near end-member beryl (Δ = 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.