• journal_title：American Mineralogist
• Contributor：Michail N. Taran ; M. Darby Dyar ; Stanislav S. Matsyuk
• Publisher：Mineralogical Society of America
• Date：2007-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2007.2163
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：92
• issue：5-6
• firstpage：753
• section：Articles

A broad (FWHM ≈ 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, 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 Fe²⁺ and Fe³⁺ in equivalent octahedral positions of structure.