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-k12 < 0.5 wt.% H2O). The IR polarization behavior in the OH stretching vibrational region of samples Sar-899, Sar-k12, 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 3618 cm−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.

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