Computational Analysis of Oxygen Isotope Effects on the Raman Spectra of Forsterite (Mg 2 SiO 4 )

The dependence of Raman spectra of forsterite (Mg 2 SiO 4 ) on oxygen isotopic ratios was systematically investigated using density functional theory (DFT) calculations with the CRYSTAL17 code . Si–O stretching vibrational frequencies and Raman intensities were computed for a series of isotopic compositions ( α = 17 O/ 16 O, 18 O/ 16 O) over the α = 0.0159–0.143 range, employing unit-cell and, for lower α values, supercell models, as well as for the end-member structures at α = 0.000 and 1.000. Partial isotopic substitution breaks the lattice symmetry, activating originally Raman-inactive modes, while site-dependent substitution induces band splitting. The simulated spectra quantitatively reproduce the experimentally observed red shifts with increasing heavy-oxygen fractions and explain the broadening of the major peaks near 810 and 960 cm –1 through a combination of symmetry reduction and site-specific effects. These results provide microscopic insight into the influence of isotopic substitution on the Raman response of forsterite and establish a theoretical foundation for future micro-Raman studies aimed at precise oxygen isotopic ratio determination.