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

Kuang Kuang,Ma Ma,Huang Huang,Li Li,Ye Ye,Xu Xu,Zhang Zhang,Zhou Zhou,Chen Chen

doi:10.3390/cryst9100541

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.

Highlights

IntroductionGarnet only one oneofofthethe most important mineral the upper an Garnetisis not most important mineral in theinupper mantlemantle [1], but [1], alsobut an important important phase in metamorphic rocks [2].garnet is stable a wide range of pressures and phase in metamorphic rocks [2]
Our results clearly show that the unit cell constants decrease with increasing Pyr contents (XPyr) in the synthetic Alm-Pyr crystals owing to cell constants decrease with increasing Pyr contents (XPyr) in the synthetic Alm-Pyr crystals owing to
Our results clearly show that the unit cell constants decrease with increasing Pyr contents (XPyr ) in the synthetic Alm-Pyr crystals owing to the smaller ionic radius of Mg2+ in eightfold coordination (0.89 Å) than that of Fe2+ (0.92 Å)

Summary

Introduction

Garnet only one oneofofthethe most important mineral the upper an Garnetisis not most important mineral in theinupper mantlemantle [1], but [1], alsobut an important important phase in metamorphic rocks [2]. Garnet is stable a wide range of pressures and phase in metamorphic rocks [2]. Garnet is stable over a wideover range of pressures and temperatures temperatures in the Earth. Garnet plays a prominent role in many geothermobarometers in the Earth. Garnet plays a prominent role in many geothermobarometers used to calculate used to calculate pressure and temperature the mantle and rock. The pressure andthe temperature of the mantle andof metamorphic rockmetamorphic [3,4]. The[3,4]

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