  Germanates are often used as structural analogs of planetary silicates. We
have explored the high-pressure phase relations in Mg$_2$GeO$_4$ using diamond
anvil cell experiments combined with synchrotron x-ray diffraction and
computations based on density functional theory. Upon room temperature
compression, forsterite-type Mg$_2$GeO$_4$ remains stable up to 30 GPa. At
higher pressures, a phase transition to a forsterite-III type (Cmc21) structure
was observed, which remained stable to the peak pressure of 105 GPa. Using a
3rd 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 the
forsterite-III type phase. The forsterite-III type structure was found to be
metastable when compared to the stable assemblage of perovskite/post-perovskite
+ MgO, as observed during laser-heating experiments. Understanding the phase
relations and physical properties of metastable phases is crucial for studying
the mineralogy of impact sites, understanding metastable wedges in subducting
slabs and interpreting the results of shock compression experiments.
