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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 2 SiO 4  and the mechanism and kinetics of its back-transformation to forsterite. Hightemperature Raman spectra of β-Mg 2 SiO 4  and its back-transformed products were recorded up to 1200 K. β-Mg 2 SiO 4  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 -1 . 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 2 SiO 4  to forsterite. Similar features are observed in the Raman spectrum of partially transformed γ-Ni 2 SiO 4  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.

american mineralogist

High-temperature Raman spectroscopic and X-ray diffraction study of β-Mg 2 SiO 4 : Insights into its high-temperature thermodynamic properties and the β- to α-phase-transformation mechanism and kinetics