1. Optical spectroscopic study of natural Fe-rich Pizzo Forno staurolite at different temperatures and pressures
- Author
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Taran, Michail N. and Koch-Muller, Monika
- Subjects
Absorption spectra -- Usage ,Materials at high pressures -- Evaluation ,Earth sciences - 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 [Fe.sup.2+] and [Ti.sup.4+] that causes a broad absorption band at ca. 25 400 [cm.sup.-1] superimposed on an intense high-energy edge. The polarization properties of the band and, also, the visible pleochroism [gamma] > [alpha] >> [beta] of the sample studied, which is found to be different to previously published data viz. [gamma] > [beta] >> [alpha], does not unambiguously give evidence of the structural positions of [Ti.sup.4+] 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 [sup.IV][Fe.sup.2+] containing three broad components at around 5445, 4625, and 3800 [cm.sup.-1] and strong narrow [beta]- and [gamma]-polarized lines of OH-vibrations in the range 3600 to 3400 [cm.sup.-1]. Curve-fitting analysis suggests that the three crystal field bands are due to lowering of the local symmetry of the tetrahedral [Fe.sup.2+] from [T.sub.d] to ca. [C.sub.2v], so they are assigned to the split electronic [sup.5]E [right arrow] [sup.5][T.sub.2] transition of [sup.IV][Fe.sup.2+]. Noticeable differences in the temperature- and pressure-induced effects on the spin-allowed bands of [sup.IV][Fe.sup.2+] in staurolite compared to spinel--another [sup.IV][Fe.sup.2+]-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 [sup.IV][Fe.sup.2+]. Keywords: Staurolite, optical absorption spectra, IVCT, dd-transitions, temperature and pressure effect DO1: 10.2138/am.2010.3207
- Published
- 2010