High-temperature Raman spectroscopic and X-ray diffraction study of β-Mg2SiO4: Insights into its high-temperature thermodynamic properties and the β- to α-phase-transformation mechanism and kinetics

• Published in: The American mineralogist 1996-05, Vol.81 (5), p.585-594
• Main Authors: Reynard, Bruno, Takir, Fouzia, Guyot, François, Gwanmesia, Gabriel D., Liebermann, Robert C., Gillet, Philippe
• Format: Article
• Language: English
• Subjects: High temperature; Kinetics; Minerals; Scientific imaging; Thermodynamics; X-ray diffraction; X-rays
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-1996-5-605

We performed in-situ Raman spectroscopy and X-ray diffraction experiments at high temperature and ambient pressure to investigate the intrinsic anharmonic properties of β-Mg SiO and the mechanism and kinetics of its back-transformation to forsterite. Hightemperature Raman spectra of β-Mg SiO and its back-transformed products were recorded up to 1200 K. β-Mg SiO persists metastably up to 800-900 K, and the Raman frequency shifts with temperature were determined. Between 800 and 1000 K, new peaks are observed at about 670 and 1020 cm . Above 1000 K, a direct transformation to forsterite occurs. The peaks that appear between 800 and 1000 K are attributed to a defective spinelloid that forms as an intermediate phase during the back-transformation of β-Mg SiO to forsterite. Similar features are observed in the Raman spectrum of partially transformed γ-Ni SiO heated at 1073 K and ambient pressure for 10 min. These results indicate that a two-step mechanism, possibly martensitic, is operative in the backtransformation of the β-and γ-phases to olivine at low to moderate temperatures and for a large overstepping of the equilibrium conditions. The kinetics of the β-to α-Mg SO 4 back-transformation were monitored between 1023 and 1120 K at ambient pressure using X-ray powder diffraction. For the kinetic data obtained in air, two regimes are evident from an Avrami analysis. The first regime is characterized by an exponent n ≈ 2 for a low transformed fraction (X < 0.5); the second has n ≈ 1 for higher transformed fractions. For this second regime, an activation energy of 432 ± 64 kl/mol is derived for the growth process from the kinetic data. A smaller data set collected in vacuum indicates much slower transformation rates and suggests a significant effect of the O or H O partial pressures on the kinetics. Intrinsic mode anharmonic parameters can be calculated from the Raman frequency shifts with temperature and used to correct vibrational heat capacity models for intrinsic anharmonic effects. These corrections are slightly higher for the β-phase than for forsterite but the difference is within the experimental error. This indicates that, within the resolution of our experiments, no significant effect of intrinsic anharmonicity on the location and slope of the α-β phase transition can be predicted.