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

TL;DR

This study investigates the high-pressure phase transition of Mg$_2$GeO$_4$ from a forsterite-type to a metastable forsterite-III structure using experiments and computations, revealing detailed equations of state and metastability conditions.

Contribution

It provides new experimental and computational insights into the phase relations and equations of state of Mg$_2$GeO$_4$ under high pressure, including the discovery of a metastable forsterite-III phase.

Findings

Forsterite-type Mg$_2$GeO$_4$ remains stable up to 30 GPa.

Phase transition to forsterite-III occurs above 30 GPa and remains stable up to 105 GPa.

Metastability of the forsterite-III phase compared to stable mineral assemblages.

Abstract

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.