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 Ia3d and unit-cell parameters a = 11.2660(2) Å and V = 1429.91(1) Å3. 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 (A1g-ν1) and O-Si-O symmetric bending (A1g-ν2) modes of LiMGt shift significantly to higher frequencies relative to the corresponding ones of pyrope. In contrast, the A1g-(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.