Crystal structure and Raman spectrum of a high-pressure Li-rich majoritic garnet, (Li₂Mg)Si₂(SiO₄)₃

A Li-rich majoritic garnet (LiMGt), (Li₂Mg)Si₂(SiO₄)₃, was synthesized at 15 GPa and 1500 °C and its structure studied with single-crystal X-ray diffraction and Raman spectroscopy. It is cubic with space group Ia3̅ and unit-cell parameters a = 11.2660(2) Å and V = 1429.91(1) ų. The 8-, 6-, and 4-coordinated cation sites in LiMGt are occupied by (Li⁺ + Mg²⁺), Si⁴⁺, and Si⁴⁺, respectively. Whereas the SiO₆ octahedron is nearly regular, the XO₈ dodecahedron is the most distorted of all known silicate garnets in terms of the bond-length distortion index. All Raman peaks of LiMGt are broader than those of pyrope, due to the substitution of Li⁺ for Mg²⁺ at the dodecahedral site. Furthermore, both Si-O symmetric stretching (A_1g-ν₁) and O-Si-O symmetric bending (A_1g-ν₂) modes of LiMGt shift significantly to higher frequencies relative to the corresponding ones of pyrope. In contrast, the A_1g-(SiO₄) rotational mode of LiMGt displays a much lower frequency than that of pyrope. This study represents the first structural report on a garnet with an all-silicate framework and suggests that, like Na incorporation in garnets, the pressure-dependent coupled substitution of (Li⁺ + Si⁴⁺) for (Mg²⁺ + Al³⁺) is likely one of the primary mechanisms for Li enrichment in garnets in the mantle and the transition zone.