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American Mineralogist; May 2007; v. 92; no. 5-6; p. 753-760; DOI: 10.2138/am.2007.2163
© 2007 Mineralogical Society of America
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Optical absorption study of natural garnets of almandine-skiagite composition showing intervalence Fe2+ + Fe3+ -> Fe3+ + Fe2+ charge-transfer transition

Michail N. Taran1,*, M. Darby Dyar2 and Stanislav S. Matsyuk1

1 Institute of Geochemistry, Mineralogy and Ore Formation, National Academy of Sciences of Ukraine, Palladin Avenue 34, Kiev-142, 252680 Ukraine
2 Mount Holyoke College, 50 College Street, South Hadley, Massachusetts 01075, U.S.A.

Correspondence: * E-mail: taran{at}igmr.relc.com

A broad (FWHM {approx} 7300 cm–1) intense band at ~21 700 cm–1 in the optical absorption spectra of natural Fe2+, Fe3+-rich garnets is attributed to electronic intervalence charge-transfer transitions (IVCT), VIIIFe2+ + VIFe3+ -> VIIIFe3+ + VIFe2+. 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 (Fe32+ Fe23+ Si3O12) 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 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 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, {Delta}{nu}/{Delta}P {approx}–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.

Key Words: Garnets • optical absorption spectra • Mössbauer spectra • iron ions • electronic dd transitions • intervalence charge-transfer transitions






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