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 4A24T1(4F) and 4A24T1(4P) of IVCo2+ are the predominant absorption features. In addition, in both cases broad bands, derived from spin-allowed electronic transitions 4E4T2 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 4A24T1(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+.

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