Single crystals of Mg2GeO4 spinel, a stable analogue to mantle ringwoodite at atmospheric pressure up to 800°C, were synthesized from Mg2GeO4 olivine in hydrous conditions at 1.9 GPa and ~1000°C. Infrared (IR) spectra show OH-stretching peaks at 3531 and 3502 cm−1. Quantification of IR spectra reveals hydrogen in the order of 5 to 10 ppm wt. H2O. A complete hydrogen/deuterium exchange was achieved at 700°C leading to a diffusion coefficient of deuterium of ~3×10−14 m2/s. This value is of the same order as diffusion coefficients observed in other mantle minerals, such as garnet and diopside.
The O–O distance of ~ 2.9 Å calculated from the spectra of Mg2GeO4 spinel is consistent with the GeO4 tetrahedra edge length known from crystal-structure data. Whereas the hydration mechanism and the site occupancies involved are still in discussion for mantle spinel, the IR measurements on Mg2GeO4 confirm the idea of protonation at the tetrahedral edges.