Crystal-Chemical Properties of Synthetic Almandine-Pyrope Solid Solution by X-Ray Single-Crystal Diffraction and Raman Spectroscopy

• Published in: Crystals (Basel) 2019-10, Vol.9 (10), p.541
• Main Authors: Kuang, Yunqian, Xu, Jingui, Li, Bo, Ye, Zhilin, Huang, Shijie, Chen, Wei, Zhang, Dongzhou, Zhou, Wenge, Ma, Maining
• Format: Article
• Language: English
• Subjects: Aluminum; Chemical properties; crystal-chemical property; Crystals; Experiments; Geochemistry; High temperature; Lasers; Magnesium aluminum silicates; Metamorphic rocks; Parameters; Raman spectra; Raman spectroscopy; Silicon; Single crystals; Software; Solid solutions; Spectrum analysis; synthetic almandine-pyrope solid solution; Tetrahedra; Unit cell; Vibration mode; Vibrational spectra; x-ray single-crystal diffraction
• ISSN: 2073-4352; 2073-4352
• DOI: 10.3390/cryst9100541

Crystal-chemical properties of synthetic Almandine-Pyrope (Alm-Pyr) solid solutions were investigated by X-ray single-crystal diffraction and Raman spectroscopy. Garnet solid solution with different compositions were synthesized from powder at 4.0 GPa and annealed at 1200 °C for 48 h by a multi-anvil pressure apparatus. Garnet crystals with different sizes (about 60–1000 μm) were obtained from synthesis. The results of X-ray single-crystal diffraction show that the unit cell constants decrease with increasing Pyr contents in the synthetic Alm-Pyr crystals due to the smaller ionic radius of Mg2+ in eightfold coordination than that of Fe2+. The data exhibit obviously positive deviations from ideal mixing volumes across the Alm-Pyr join which may be caused by the distortion of the SiO4 tetrahedron. Moreover, the significant decrease in the average M-O bond length and volume of the [MgO8]/[FeO8] dodecahedron with increasing Pyr contents are the most important factors to the decrease in the Alm-Pyr crystal unit cell constant and volume. On the other hand, selected bond distances (average , , and distances) have a linear correlation with the unit-cell parameter, but the distance has nonlinear correlation. With increasing the unit-cell parameter, the average distance increases significantly, followed by the average and distances. While the distance changes negligibly further confirming the conclusion that the significant decrease of the average M-O bond length of the [MgO8]/[FeO8] dodecahedron with increasing Pyr contents are the most important factors to the decrease in the Alm-Pyr crystal unit cell volume. In the Raman spectra collected for the Alm-Pyr solid solutions, Raman vibration mode assignments indicate that the Raman vibrational spectra change along the Alm-Pyr binary solution. The mode frequencies of Si-O stretching, Si-O bending, and the rotation of the SiO4-tetrahedron (R(SiO4)) decrease linearly, while the translational modes of the SiO4-tetrahedron (T(SiO4)) increase with increasing Alm contents.