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Germanates are often used as structural analogs of planetary silicates. Wehave explored the high-pressure phase relations in Mg$_2$GeO$_4$ using diamondanvil cell experiments combined with synchrotron x-ray diffraction andcomputations based on density functional theory. Upon room temperaturecompression, forsterite-type Mg$_2$GeO$_4$ remains stable up to 30 GPa. Athigher pressures, a phase transition to a forsterite-III type (Cmc21) structurewas observed, which remained stable to the peak pressure of 105 GPa. Using a3rd order Birch Murnaghan fit to the experimental data, we obtained V0 = 305.1(3) {\AA}3, K0 = 124.6 (14) GPa and K0' = 3.86 (fixed) for forsterite- and V0 =263.5 (15) {\AA}3, K0 = 175 (7) GPa and K0' = 4.2 (fixed) for theforsterite-III type phase. The forsterite-III type structure was found to bemetastable when compared to the stable assemblage of perovskite/post-perovskite+ MgO, as observed during laser-heating experiments. Understanding the phaserelations and physical properties of metastable phases is crucial for studyingthe mineralogy of impact sites, understanding metastable wedges in subductingslabs and interpreting the results of shock compression experiments.

High-pressure Phase Transition of Olivine-type Mg$_2$GeO$_4$ to a Metastable Forsterite-III type Structure and their Equation of States