Raman spectroscopic study of liebenbergite and Ni₂SiO₄ spinel at high pressure and high temperature: nickel effects on the vibration properties of olivine and spinel structures

High-pressure and high-temperature Raman spectroscopic measurements of synthetic liebenbergite and Ni₂SiO₄ spinel have been conducted up to 22 GPa and 700 ℃, respectively. Isothermal and isobaric mode Grüneisen parameters were calculated based on the observed Raman modes. The intrinsic anharmonicities of liebenbergite and Ni₂SiO₄ spinel were also evaluated. The changes of the asymmetric SiO₄ stretching band of Ni₂SiO₄ spinel in frequency are irreversible under decompression, indicating a potential pressure-induced modification in the crystal structure at elevated pressures. The values of isothermal mode Grüneisen parameters show that the SiO₄ internal vibrations in Ni-rich olivines are more sensitive to the variations of pressure. For spinel-group minerals, the SiO₄ internal vibrations can be less sensitive to the pressure change due to nickel incorporation. In contrast, according to the values of isobaric mode Grüneisen parameters, nickel increases the sensitivity of these vibrations to the variations of temperature. In addition, nickel has distinctive effects on the intrinsic anharmonicities of different vibration modes in both olivine and spinel-group minerals, and therefore alter the thermodynamic properties of their crystal structures.