• journal_title：American Mineralogist
• Contributor：Michail N. Taran ; Monika Koch-Müller ; Anne Feenstra
• Publisher：Mineralogical Society of America
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2009.3247
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：94
• issue：11-12
• firstpage：1647
• section：Articles

Optical absorption spectra of natural Co-bearing spinel and staurolite were studied at different temperatures and pressures. In both minerals, two broad, intense structured bands in the range 5500–8000 and 15 000–19 000 cm⁻¹, caused by electronic spin-allowed transitions ⁴A₂ → ⁴T₁(⁴F) and ⁴A₂ → ⁴T₁(⁴P) of ^IVCo²⁺ are the predominant absorption features. In addition, in both cases broad bands, derived from spin-allowed electronic transitions ⁴E → ⁴T₂ of ^IVFe²⁺, appear in the near infrared range partly overlapping the bands caused by ^IVCo²⁺. In staurolite the NIR range of the spectra are complicated by intense sharp lines of OH-vibrations at around 3400 cm⁻¹.

In spinel, with a regular tetrahedral site, the splitting of the spin-allowed bands I and II of ^IVCo²⁺ is assumed to be caused by spin-orbit and vibronic coupling. In staurolite, the splitting is stronger due to the additional low-symmetry crystal field effect of ^IVCo²⁺. It is found that the effect of temperature and pressure on the behavior of the ⁴A₂ → ⁴T₁(⁴P) bands of ^IVCo²⁺ in the two minerals are rather similar, in contrast to our findings for the spin-allowed bands of ^IVFe²⁺ in spinel and staurolite. This is interpreted as a manifestation of a dynamic Jahn-Teller effect for ^IVFe²⁺ and lack of it in case of ^IVCo²⁺.