Optical absorption study of natural garnets of almandine-skiagite composition showing intervalence Fe2+ + Fe3+ -> Fe3+ + Fe2+ charge-transfer transition

A broad (FWHM ≈ 7300 cm -1 ) intense band at ~21 700 cm -1 in the optical absorption spectra of natural Fe 2+ , Fe 3+ -rich garnets is attributed to electronic intervalence charge-transfer transitions (IVCT), VIIIFe 2+ + VIFe 3+ → VIIIFe 3+ + VIFe 2+ . In Fe 3+ , Fe 2+ -bearing garnets of predominantly almandine compositions, this band causes yellowish tinges in addition to the pink color, typical of pure Fe 3+ -free almandines. In garnets from deeper-seated mafic granulites from kimberlite pipes in Siberia with high skiagite (Fe 3 2+ Fe 2 3+ Si 3 O 12 ) contents, IVCT causes intense brownish-yellow colors. The relatively high energy of the band (~21 700 cm -1 ) compared to diverse minerals showing IVCT between Fe 2+ and Fe 3+ in adjacent octahedral sites, is attributed to the charge-transfer transition taking place between Fe 2+ and Fe 3+ in non-equivalent, dodecahedral and octahedral sites of the garnet structure. Band intensity is directly correlated with the product of Fe 2+ and Fe 3+ as measured by Mössbauer spectroscopy. The energy of the IVCT band is nearly independent of temperature, whereas its intensity decreases slightly with increasing temperature. Pressure induces a weak shift of the band to lower energies, Δν/ΔP ≈ -75 cm -1 /GPa, but intensity of the bands remains practically unchanged. Such temperature and pressure dependencies are quite different from those in other minerals showing IVCT between Fe 2+ and Fe 3+ in equivalent octahedral positions of structure.