High-pressure Raman scattering study of majorite-Garnet solid solutions in the system Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂

We report here the results of Raman scattering measurements to 30 GPa on polycrystalline end-member Mj₁₀₀ tetragonal majorite and two majorite-garnet solid solutions, Mj₅₀Py₅₀ and Mj₅₀Py₂₀ (where Mj = Mg₄Si₄O₁₂ and Py = Mg₃Al₂Si₃O₁₂), synthesized at 17.7 GPa and 2000°C. The results are discussed in light of previous Raman, X ray, and elasticity studies. At ambient pressure, variations in the Raman spectra indicate changes in the structure and disorder with increasing Py content. For example, the spectra of Mj₁₀₀ confirm its tetragonal structure, and the two bands in Mj₁₀₀ at ~885 cm^-1 and ~928 cm^-1 merge in the case of Mj₈₀Py₂₀ and Mj₅₀Py₅₀- The spectra of Mj₅₀Py₅₀ and Mj₈₀Py₂₀ show similarity to those of Py₁₀₀ (cubic garnet structure) and (Mg_.79Fe_.21)₄Si₄O₁₂ majorite from the Catherwood meteorite, which also has cubic structure. The pressure dependencies of the Raman frequencies for all the modes are found to be linear, without discontinuities, throughout the pressure range of this study, implying no major structural changes. Using the pressure shift of the Raman bands, (∂v_i/∂P), the mode Griineisen parameters, γ_i, are calculated using the bulk modulus values measured by Brillouin spectroscopy. The averaged values of mode γ_i for the three Mj-Py samples lie in the range 0.81-1.13, lower than those of pyrope garnet (1.5) but close to γ_th ~1.1 calculated for Mj₅₈Py₄₂.