Optical spectroscopic study of tetrahedrally coordinated Co2+ in natural spinel and staurolite at different temperatures and pressures

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 4 A 2 → 4 T 1 (4F) and 4 A 2 → 4 T 1 ( 4 P) of IV Co 2+ are the predominant absorption features. In addition, in both cases broad bands, derived from spin-allowed electronic transitions 4E → 4T2 of IV Fe 2+ , appear in the near infrared range partly overlapping the bands caused by IV Co 2+ . 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 IV Co 2+ 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 IV Co 2+ . It is found that the effect of temperature and pressure on the behavior of the 4 A 2 → 4 T 1 ( 4 P) bands of IV Co 2+ in the two minerals are rather similar, in contrast to our findings for the spin-allowed bands of IV Fe 2+ in spinel and staurolite. This is interpreted as a manifestation of a dynamic Jahn-Teller effect for IV Fe 2+ and lack of it in case of IV Co 2+