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

Abstract

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 <M-O>, <Al-O>, and <D-O> distances) have a linear correlation with the unit-cell parameter, but the <Si-O> distance has nonlinear correlation. With increasing the unit-cell parameter, the average <M-O> distance increases significantly, followed by the average <D-O> and <Al-O> distances. While the <Si-O> 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.

Raman Microspectroscopy of Garnets from S-Fibulae from the Archaeological Site Lajh (Slovenia)

“…Anyway, some bands, for instance, Raman band at 214 cm −1 (band II) assigned to the translational mode of the SiO 4 tetrahedron, showed the opposite trend with increasing the mode frequencies with almandine contents. An inverse correlation with ionic radii Mg 2+ (0.89) < Fe 2+ (0.92) < Mn 2+ (0.96) Å for this frequency mode was reported also by other authors [22,26]. Figure 7 shows trends in the selected frequency distribution from the four ranges of the Raman spectra assigned to the translation mode of the X 2+ cation (band I), rotation of the SiO 4 tetrahedron (band V) and vibration modes of Si-O bending (ν 4 -band XII) and Si−O stretching (ν 3 -band XV).…”

“…Raman bands (cm −1 ) for the investigated garnets, assigned to site motion and mode of vibration. The Raman spectra of investigated garnets agree well with Raman spectra of pyralspites and the dominant end member almandine [20][21][22]26], and can be grouped into three distinct energy regions: low frequency regions between (169-374 cm −1 ); medium energy bands (479-637 cm −1 ); and high energy peaks (862-1046 cm −1 ) ( Figure 3). Among the expected 25 Raman active modes (3A 1g + 8E g + 14T 2g ) of garnet group minerals [20], a total of 15 vibration modes were observed in the Raman spectra obtained that could be grouped into four main regions [22].…”

“…Similarly, Kuang et al (2019) [26] observed a linear increase in the frequencies of Si-O stretching and bending, the rotation mode of the tetrahedron (R(SiO 4 )) and the translation of the X 2+ mode (T(X 2+ )) with increasing pyrope content and vice versa for the translation of the tetrahedron (T(SiO 4 )) when studying synthetic pyrope-almandine garnets, which is consistent to a certain extent (considering the vibration modes) with our results.…”

“…However, shifts of the Raman bands to higher frequencies, e.g., modes assigned to the translation mode of X 2+ , the rotation mode of SiO 4 tetrahedron and the Si-O stretching mode, indicate the presence of higher pyrope content in the solid solution for the three garnets. The correlation between the composition of the natural garnets and changes in Raman vibrations was determined in the study of Henderson (2009) [11,22,26]. In the case of almandine garnets, Henderson (2009) [11] observed shifts to higher frequencies (the Si-O bending (ν 3 ) at 1044-1047 cm −1 , the stretching (ν 4 ) 636-641 cm −1 and the rotation (R(SiO 4 )) modes at 350-357 cm −1 ) with an increase in the pyrope content, although when spessartine content increased in composition, this end member interfered with the linear trend of the increase in all mode frequencies.…”

“…Thus, the shift of a Raman band due to a chemical substitution is a useful indicator of chemical composition and instead of observing Raman vibrations of these specific elements, one can conveniently observe the shifting of the numerous types of Si-O vibrations within or between the tetrahedral sites [21][22][23]. For instance, Raman mode frequencies of garnets along the almandine-pyrope solid solution series show the vibrational spectra change with the composition, where the Si-O stretching, Si-O bending, and the rotation of the SiO 4 tetrahedron (R(SiO 4 )) mode frequencies decrease linearly, but the translational mode frequencies of the SiO 4 tetrahedron (T(SiO 4 )) increase with the almandine content [26].Inclusions are yet another useful tool in interpreting the geological environment in which garnets are formed, although this study area is still under development [27]. Inclusions are either minerals, melts or fluids that can be trapped or formed in various phases of mineral formation [28].…”