Ultrahigh-pressure disordered eight-coordinated phase of Mg ₂ GeO ₄ : Analogue for super-Earth mantles

Abstract

Mg2GeO4 is important as an analog for the ultrahigh-pressure behavior of Mg2SiO4, a major component of planetary interiors. In this study, we have investigated magnesium germanate to 275 GPa and over 2,000 K using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction and density functional theory (DFT) computations. The experimental results are consistent with the formation of a phase with disordered Mg and Ge, in which germanium adopts eightfold coordination with oxygen: the cubic, Th3P4-type structure. DFT computations suggest partial Mg-Ge order, resulting in a tetragonal I4¯2d structure indistinguishable from I4¯3d Th3P4 in our experiments. If applicable to silicates, the formation of this highly coordinated and intrinsically disordered phase may have important implications for the interior mineralogy of large, rocky extrasolar planets.

Synthesis and Stability of an Eight‐Coordinated Fe3O4 High‐Pressure Phase: Implications for the Mantle Structure of Super‐Earths

“…The 2 M E uncertainty in the lower limit for MgSiO 3 breakdown in these models is based on the varying calculation results on the breakdown products and pressures for MgSiO 3 into Mg‐silicates (Umemoto et al., 2017) versus oxides (Tsuchiya & Tsuchiya, 2011). Considering this work and recent post‐post spinel analog studies (Dutta et al., 2022; Khandarkhaeva et al., 2021), Mg 2 SiO 4 is a breakdown product at lower pressures than the formation of the MgO and SiO 2 oxides, following Umemoto et al. (2017) (Figure 4).…”

“…Whereas the mantle structures of Earth and the terrestrial planets are characterized by phase transitions in silicates associated with increases in Si coordination number from 4 to 6, further densification transitions are predicted in the interior of super‐Earths (Tsuchiya & Tsuchiya, 2011; Umemoto & Wentzcovitch, 2019; Umemoto et al., 2017). MgSiO 3 post‐perovskite is predicted to break down above 500 GPa and, of the possible breakdown products, the current study along with recent analog studies suggests that the Th 3 P 4 ‐related Mg 2 SiO 4 is likely to form with Si in 8‐fold coordination (Dutta et al., 2022; Khandarkhaeva et al., 2021; Umemoto & Wentzcovitch, 2019; Umemoto et al., 2017). A schematic of a super‐Earth interior compiling the known and predicted Mg‐silicate transitions is shown in Figure 4.…”

“…While this density transition would not be resolvable from the mass‐radius relationships observed for exoplanets, one can gauge from the effects of changes in mineralogy and density in the layered structure of Earth's mantle, that this post‐post spinel transition could have profound effects on the chemical, elastic, rheological, and dynamic properties of super‐Earth lower mantles. For example, both Fe 2+ and Fe 3+ occupy equivalent 8‐fold coordinated sites in δ‐Fe 3 O 4 , and recent experimental and ab‐initio studies suggests that Mg 2+ and Si 4+ also disorder in the post‐post spinel (ppSp) structure with increasing temperature (Dutta et al., 2022; Umemoto & Wentzcovitch, 2021). As the difference in cation radii of Mg 2+ and Si 4+ are greater than that of Mg 2+ and Fe, 3+ Fe 3+ is likely to readily partition into the Th 3 P 4 ‐type Mg 2 SiO 4 making this 8‐fold Mg‐silicate an important iron‐bearing phase in super‐Earth mantles.…”

“…The measured masses and radii of super‐Earths, however, cannot be used to determine a unique solution for their internal structures and core‐mass fractions (CMFs). The possible range of super‐Earth interior density, thermal, and dynamic structures and the effect on their observable properties have been the subject of recent experimental and computational studies (e.g., Blaske & O’Rourke, 2021; Boujibar et al., 2020; Driscoll & Olson, 2011; Dutta et al., 2022; Fei et al., 2021; Tsuchiya & Tsuchiya, 2011; Umemoto et al., 2017).…”

A high-pressure phase of Na2CuF4 with eight-coordinated Cu2+ – a low-pressure analogue of Mg2SiO4

“…Recent diamond anvil cell (DAC) experiments on Mg 2 GeO 4 confirmed the formation of a Th 3 P 4 -like phase upon laser heating at 160 GPa. 12 However, the quality of the measured X-ray diffraction profile was insufficient to determine whether Mg 2 GeO 4 adopts the fully-disordered I 4̄3 d structure or a partially disordered phase. A similar Th 3 P 4 -related structure was observed above 60 GPa in laser-heated DAC experiments on Fe 3 O 4 .…”