The crystal chemistry of birefringent natural uvarovites: Part I. Optical investigations and UV-VIS-IR absorption spectroscopy

Six garnet crystals with compositions close to the uvarovite-grossular bina ry from three different localities (Saranov, Veselovsk, and Saranka, Ural M ountains, Russia) were investigated by optical methods, electron microprobe analysis, and polarized UV-VIS-IR micro-spectroscopy. Under crossed polari zers the crystals display "strong" birefringence in the order of 0.006 and formation of domains, aligned parallel to the crystal faces of the form {11 0}. Referring to the cubic cell, Y is always parallel to a principal lattic e direction, [010](cub), whereas X and Z are then parallel to [101](cub) an d [10 (1) over bar](cub), respectively. The garnets are uvarovite-grossular solid solutions with 48 to 71 mol% uvar ovite component and traces of Ti, Fe, Mn, and Mg, and exhibit no compositio nal zoning. Polarized single crystal infrared absorption spectra in the region of the O H stretching vibration display 14 discernible, partly superimposing, and st rongly anisotropic absorption bands between 3470 and 3680 cm(-1), which are caused by structurally incorporated hydroxyl groups. The polarization beha vior observed for individual samples complies with orthorhombic, monoclinic , and triclinic crystal symmetry, respectively. The water content calculate d using an integral extinction coefficient, epsilon = 43800 L/cm(2). mol, r anges from 0.07 to 0.34 wt%. Annealing experiments indicate that the incorp oration of hydroxyl groups is not the primary cause for the anisotropic beh avior. Instead, cation ordering on octahedral positions may play an importa nt role. This assumption is substantiated by single crystal X-ray diffracti on investigations reported in Part II of the present study (Wildner and And rut 2001). Contrary to the IR spectroscopic data, single-crystal electronic absorption spectra show no significant polarization behavior in the UV-VIS-NIR spectr al region. The observed absorption bands are caused by d-d transitions of C r3+ cations in octahedral coordination. The linear extinction coefficients of the investigated uvarovites strongly indicate that the Cr3+ cations occu py centrosymmetric sites, regardless of the exact symmetry reduction from I a (3) over bard.