Cation Disorder of Mg2SiO4 $\textMg_\mathbf2\textSiO_\mathbf4$ in Super-Earth Mantles

DH Zheng and YF Li and YH Peng and RS Gong and ZK Liu and J Deng, GEOPHYSICAL RESEARCH LETTERS, 52, e2025GL118419 (2025).

DOI: 10.1029/2025GL118419

Understanding the mineralogy of exoplanets is essential for unraveling their interior structures, dynamics, and evolution. For large super- Earths, the post-post spinel , one of the major mantle phases, may undergo the order-disorder transition (ODT) at high temperatures. However, the ODT phase boundary of has not been rigorously constrained. Additionally, fundamental thermodynamic properties of the disordered remain poorly investigated. Here, we develop a unified machine learning potential (MLP) for of ab initio accuracy under super-Earth mantle conditions. With the efficient MLP, we extensively calculate the free energy of post-post spinel via the thermodynamic integration method. The results are used to constrain the ODT phase boundary. Furthermore, we report the P-V-T equation of state and Gr & uuml;neisen parameters for post-post spinel across various degrees of disorder. These thermodynamic properties are further applied to update the adiabatic thermal profiles and the mass-radius relation of super-Earths.

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