Raman spectroscopic study of synthetic pyrope–grossular garnets: structural implications

A study of the effect of substitution of Mg and Ca in garnet solid solution (Grt_ss) was carried out using Raman spectroscopy to probe changes to the crystal lattice. The garnet solid solutions with composition changing along pyrope (Py; Mg₃Al₂Si₃O₁₂) and grossular (Gr; Ca₃Al₂Si₃O₁₂) binary were synthesized from glass at 6 GPa and 1400 °C and a second series of Grt_ss with composition Py₄₀Gr₆₀ were synthesized at 6 GPa but different temperatures from 1000 to 1400 °C. Raman mode assignments were made based on a comparison with the two end members pyrope and grossular, which show consistent result with literature study on single crystals data. The correlation between the Raman mode frequencies and compositional changes along the pyrope–grossular binary suggests a two-mode behavior for Mg and Ca cations in the garnet structure. The full widths at half-maximum of selected Raman modes increase on moving away from the end members and are about double the end-member values in the mid-position, where the frequencies closely linearly change with composition. The frequencies of the translational modes of the SiO₄ tetrahedron (T(SiO₄)) show large deviations from linearity indicating a strong kinematic coupling with the translational modes of the Ca and Mg cations. The anomalies in T(SiO₄) are linked to mixing unit cell volume, suggesting that the nonlinear mixing volume behavior along the pyrope–grossular binary is related to the resistance of the Si–O bond to expansion and compression, which is caused by substitution of Mg and Ca cations in the dodecahedral sites. Annealing temperature also shows effect on Raman mode frequencies, but the main factor controlling the changes in mode frequencies along pyrope–grossular binary is composition.