Polarized absorption spectra of trivalent chromium in forsterite, Mg2SiO4

The polarized absorption spectra of trivalent chromium in forsterite (Mg₂SiO₄) were studied at room and liquid nitrogen temperature covering the range from roughly 4 000 to 44 000 cm⁻¹. The spectra show only one absorption in each polarization direction. The γ-polarized bands at 17 500 cm⁻¹ are strongest and well resolved whereas the α- and β-polarized bands at 13 500 and 15 100 cm⁻¹, respectively, are less intensive, broadened but also structured. All absorption bands exhibit satellites spaced by approx. 730 cm⁻¹. Between ca. 19 000 and 32 000 cm⁻¹ no absorption has been observed. The interpretation of the spectra is based on the assumption that the absorption is mainly due to Cr³⁺-ions at M2 sites. The local symmetry of the Cr³⁺ (M2) complex is described by the orthorhombic point group D₂. The D₂ symmetry and also the absence of any absorption between 19 000 and 32 000 cm⁻¹ is discussed on the basis of a strong structural distortion of the Cr³⁺ (M2) complex in comparison to the M2 coordination in the pure Mg₂SiO₄ structure. The observed satellites of the absorption bands are assumed to arise from vibrations of the CrO₆ complex.