Relationship between Raman spectral pattern and crystallographic orientation of a rock-forming mineral: A case study of Fo₈₉Fa ₁₁ olivine

The relationship between Raman spectra and crystallographic orientation was examined for single crystals of Fo₈₉Fa₁₁ olivine [(Mg_0.89Fe_0.11)₂SiO₄]. Raman spectra were obtained for chemically homogeneous olivine grains with various orientations on a thin section of mantle-derived rock (dunite) using micro-Raman equipment and an unpolarized exciting laser. Crystallographic orientations of each olivine grain were determined using an electron backscattered diffraction (EBSD) method. Five apparent peaks at 822 (peak 1), 854 (peak 2), 880 (peak 3), 917 (peak 4), and 959 cm^-1 (peak 5) were observed in the spectral range of 700-1050 cm^-1. Intensity ratios of peak i to peak 2, I _i/I₂, for i = 1, 4, and 5 were formulated empirically as functions of crystallographic orientations: I_i/I₂ = (a_1i θ²/π² + a_2i θ/π + a_3i) sin² φ + (b_1i θ²/π² + b_2i θ/π + b _3i) sin φ + c_i where a_1i, a_2i, a_3i, b_1i, b_2i, b_3i, c_i are constants. φ is the angle between the [100] axis and the incident direction of the laser, and θ is the angle between the [001] axis and the incident direction of laser projected on the {100} plane. These equations well describe the relationships between I_i/I₂ and crystallographic orientation. The obtained empirical equations enable Raman spectroscopic determination of the crystallographic orientation of olivine.