Phase equilibria of (Mg,Fe)(2)SiO4 at the Earth%26apos;s upper mantle conditions from first-principles studies

作者：Yu Yonggang G^*; Vinograd Victor L; Winkler Bjoern; Wentzcovitch Renata M

来源：Physics of the Earth and Planetary Interiors, 2013, 217: 36-47.

DOI：10.1016/j.pepi.2013.01.004

• Fe2SiO4
• Fayalite
• Wadsleyite
• Ringwoodite (gamma-spinel)
• Phase equilibrium
• Thermodynamic properties
• 410-km and 520-km discontinuity
• LDA plus U

Phase equilibria of alpha, beta, and gamma (Mg,Fe)(2)SiO4 are important to understanding the mineralogy of the Earth%26apos;s upper mantle. Using the first principles approach, we studied thermodynamic properties and phase stability fields of Fe2SiO4. We show that the correct phase transition sequence in Fe2SiO4 (alpha -%26gt; gamma) can be obtained with the DFT + self-consistent Hubbard U method, while standard DFT methods (LSDA and sigma-GGA) as well as the DFT + constant U method fail the task. The vibrational virtual crystal approximation was used to derive the phonon density of state of the Fe2SiO4 polymorphs. High-pressure thermodynamic properties of Fe2SiO4 are then derived with the aid of the quasi-harmonic approximation. They are in very good agreement with experiments. The phase diagram of the (Mg,Fe)(2)SiO4 system is calculated under the assumption of ideal mixing within alpha, beta, and gamma solid solutions. The model permits the investigation of the temperature and pressure effects on the phase boundaries. The widths of the divariant alpha-beta and beta-gamma loops are barely sensitive to temperature between 1473 and 1873 K. This study shows the promise of applying the DFT + self-consistent Hubbard U method to study phase equilibria of iron-bearing Earth minerals.

• 出版日期2013-4