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

摘要

A broad (FWHM approximate to 7300 cm(-1)) intense band at similar to 21 700 cm(-1) in the optical absorption spectra of natural Fe2+, Fe3+-rich garnets is attributed to electronic intervalence charge-transfer transitions (IVCT), Fe-VIII(2+) + Fe-VI(3+) -> Fe-VIII (3+) + Fe-VI(2+). In Fe3+, Fe2+-bearing garnets of predominantly almandine compositions, this band causes yellowish tinges in addition to the pink color, typical of pure Fe3+-free almandines. In garnets from deeper-seated mafic granulites from kimberlite pipes in Siberia with high skiagite (Fe-3(2+),Fe-2(3+) Si3O12) contents, IVCT causes intense brownish-yellow colors. The relatively high energy of the band (similar to 21 700 cm-1) compared to diverse minerals showing IVCT between Fe2+ and Fe3+ in adjacent octahedral sites, is attributed to the charge-transfer transition taking place between Fe2+ and Fe3+ in non-equivalent, dodecahedral and octahedral sites of the garnet structure. Band intensity is directly correlated with the product of Fe2+ and Fe3+ as measured by Mossbauer 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, Av/AP approximate to -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 Fe2+ and Fe3+ in equivalent octahedral positions of structure.