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

We performed in-situ Raman spectroscopy and X-ray diffraction experiments at high temperature and ambient pressure to investigate the intrinsic anharmonic properties of beta-Mg2SiO4 and the mechanism and kinetics of its back-transformation to forsterite. High-temperature Raman spectra of beta-Mg2SiO4 and its back-transformed products were recorded up to 1200 K. beta-Mg2SiO4 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 beta-Mg2SiO4 to forsterite. Similar features are observed in the Raman spectrum of partially transformed gamma-Ni2SiO4 heated at 1073 K and ambient pressure for 10 min. These results indicate that a two-step mechanism, possibly martensitic, is operative in the back-transformation of the beta- and gamma-phases to olivine at low to moderate temperatures and for a large overstepping of the equilibrium conditions.