Abstract

Polarized spectra of natural brownish-yellow Fe-rich staurolite from Pizzo Forno (Switzerland) evidence that its color is very likely due to an intervalence charge-transfer transition between Fe2+ and Ti4+ that causes a broad absorption band at ca. 25 400 cm−1 superimposed on an intense high-energy edge. The polarization properties of the band and, also, the visible pleochroism γ > α >> β of the sample studied, which is found to be different to previously published data viz. γ > β >> α, does not unambiguously give evidence of the structural positions of Ti4+ ions involved in the IVCT process. In the NIR-range, we observe a broad structured absorption envelope caused by an electronic spin-allowed band of IVFe2+ containing three broad components at around 5445, 4625, and 3800 cm−1 and strong narrow β- and γ-polarized lines of OH-vibrations in the range 3600 to 3400 cm−1. Curve-fitting analysis suggests that the three crystal field bands are due to lowering of the local symmetry of the tetrahedral Fe2+ from Td to ca. C2v, so they are assigned to the split electronic 5E5T2 transition of IVFe2+. Noticeable differences in the temperature- and pressure-induced effects on the spin-allowed bands of IVFe2+ in staurolite compared to spinel—another IVFe2+-bearing mineral—are observed. This implies a suppression of the Jahn-Teller effect in staurolite that is normally intrinsic to ions with doublefold degenerate electronic ground state such as IVFe2+.

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