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American Mineralogist; November-December 2009; v. 94; no. 11-12; p. 1647-1652; DOI: 10.2138/am.2009.3247
© 2009 Mineralogical Society of America
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Right arrow Articles by Taran, M. N.
Right arrow Articles by Feenstra, A.

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

Michail N. Taran1,*, Monika Koch-Müller2,{dagger} and Anne Feenstra{ddagger}

1 Institute of Geochemistry, Mineralogy and Ore Formation, National Academy of Science of Ukraine, Palladin Avenue, 34, 03680 Kyiv-142, Ukraine
2 Deutsches GeoForschungsZentrum, Sektion 3.3, Telegrafenberg, 14473 Potsdam, Germany

Correspondence: * E-mail: m_taran{at}hotmail.com

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+.

Key Words: Spinel • staurolite • tetrahedral coordination • Co2+ • optical absorption spectra • temperature and pressure effects






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