High-Magnesium Olivines (Fo 90–100): Determination of Fo Content by Raman Spectroscopy

Olivine is a dominant constituent of the Earth’s upper mantle, and its forsterite content (Fo = 100 × Mg/(Mg + Fetotal) in molar basis) holds significant implications for indicating petrogenesis. The characteristic Raman doublet near ~820 and ~855 cm−1 shifts systematically to higher wavenumbers with increasing Fo content. Although previous studies have established general relationships between Fo content and Raman shifts in olivine, research focusing specifically on high-Fo (90–100) compositions remains limited, primarily due to a scarcity of suitable samples. This study addresses this gap by systematically investigating 45 high-Fo (90–100 olivine samples, to establish regression relationships between the Fo content and both the primary doublet (P1: 822–826 cm−1; P2: 855–858 cm−1) and three secondary peaks (P3: 881–884 cm−1, P4: 917–921 cm−1, and P5: 961–967 cm−1). Our results show that, whereas the secondary peaks (P3–P5) show weak correlations with Fo values, the doublet exhibits a strong compositional dependence, providing a reliable basis for developing calibration models. To enable the rapid screening of unknown olivines, we established a generalized linear equation (Fo = −(3547 ± 65) + (4.25 ± 0.08) P2), with P2 > 855.0 cm−1 indicating Fo > 90. For the precise quantification of these identified high-Fo samples, calibration models derived from the doublet show an excellent correlation with Fo (R2 > 0.93), with residual fluctuation within ±2.5%, a leave-one-out cross-validation root-mean-square error (LOOCV-RMSE) of ~0.7. Notably, the quadratic regression model based on the P2 peak, Fo = (346,357 ± 10,890) − (812.4 ± 287.8) P2 + (0.477 ± 0.028) P22, demonstrates exceptional predictive stability and generalization capability, with prediction errors constrained within 4 Fo units. This model provides a reliable tool for the compositional discrimination for high-Fo olivine, enriches the Raman spectral database for olivine studies, and offers a robust method for the rapid and accurate compositional analysis of both terrestrial and extraterrestrial olivine samples.