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The crystal chemistry of birefringent natural uvarovites. Part IV. OH defect incorporation mechanisms in non-cubic garnets derived from polarized IR spectroscopy

Institut für Mineralogie und Kristallographie, Universität Wien - Geozentrum, Althanstraße 14, A-1090 Wien, Austria

^* Corresponding author, e-mail: manfred.wildner{at}univie.ac.at

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.

Key-words: garnets, uvarovite, IR spectroscopy, OH defects, hydrogarnet substitution..

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