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

The dependence of Raman spectra of forsterite (Mg2SiO4) 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 = 17O/16O, 18O/16O) 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.