• journal_title：American Mineralogist
• Contributor：Michail N. Taran ; Monika Koch-Müller
• Publisher：Mineralogical Society of America
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2010.3207
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：95
• issue：2-3
• firstpage：323
• section：Articles

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 T_d to ca. C_2v, so they are assigned to the split electronic ⁵E → ⁵T₂ 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⁺.