The crystal chemistry of birefringent natural uvarovites. Part IV. OH defect incorporation mechanisms in non-cubic garnets derived from polarized IR spectroscopy
Andrut, Michael; Wildner, Manfred; Beran, Anton
European Journal of Mineralogy Volume 14 Number 6 (2002), p. 1019 - 1026
published: Dec 1, 2002
ArtNo. ESP147051406004, Price: 29.00 €
The anisotropic OH stretching vibrational behavior of three birefringent garnet crystals with compositions close to the uvarovite-grossular binary from the locality Saranov (Ural Mountains, Russia) are investigated in detail by polarized IR micro-spectroscopy. These garnet samples exhibit low but significantly different water contents (Sar-899 < sar-w2 < sar-kl2 < 0.5 wt.% h2O). The IR polarization behavior in the OH stretching vibrational region of samples Sar-899, Sar-kl2, and Sar-w2 complies with orthorhombic, monoclinic, and triclinic crystal symmetry, respectively. The corresponding spectra consist of 14 discernible, partly superimposing absorption bands between 3680 and 3470 cm-1, which are assigned to structurally incorporated hydroxyl groups. According to the individual pleochroic behavior of ten clearly non-isotropic bands, six different pleochroic patterns, i.e. four band doublets and two single bands are distinguished. For the band doublets at 3559/3540 cm-1, 3572/3565 cm-1, and 3595/3588 cm-1, as well as for the single 3618cm-1 band, models for a structural OH incorporation based on the classical (O4H4) hydrogarnet substitution are proposed. In contrast, for the band doublet at 3652/3602 cm-1 and the single band at 3640 cm-1, OH defect incorporation is explained assuming the presence of vacancies on octahedral or dodecahedral cation positions, leading to [SiO3(OH) tetrahedral groups. It is concluded that in garnets with low water contents the [SiO3(OH)] substitution plays an essential role as OH defect incorporation mechanism.