High-pressure phase transition of olivine-Type Mg2GeO4to a metastable forsterite-III type structure and their equations of state

Germanates are often used as structural analogs of planetary silicates. We have explored the high-pressure phase relations in Mg2GeO4 using diamond-Anvil cell experiments combined with synchrotron X-ray diffraction and computations based on density functional theory. Upon room temperature compression, forsterite-Type Mg2GeO4 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 third-order Birch Murnaghan fit to the experimental data, we obtained V0 = 305.1(3) Å3, K0 = 124.6(14) GPa, and K0′ $\begin{array}{} \displaystyle K_{0}^{\prime} \end{array}$ = 3.86 (fixed) for forsterite-Type Mg2GeO4 and V0 = 263.5(15) Å3, K0 = 175(7) GPa, and K0′ $\begin{array}{} \displaystyle K_{0}^{\prime} \end{array}$ = 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.