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European Journal of Mineralogy; September, October 2006; v. 18; no. 5; p. 653-664; DOI: 10.1127/0935-1221/2006/0018-0653
© 2006 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
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Articles

Characterization of pyrrhotite in Cu-Ni-ore bodies from mines in Botswana by Mössbauer spectroscopy, X-ray diffraction, and thermomagnetometry

Embaie FERROW1,*, Jacob ADETUNJI2 and John S. NKOMA3

1 GeoBiosphere Science Centre, Department of Geology, Mineralogy & Petrology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden
2 Division of Earth Systems Science, University of Derby, Kedleston Road, Derby, DE22 1 GB UK
3 Department of Physics, University of Botswana, Private Bag 0022, Gaborone, Botswana

* E-mail: embaie.ferrow{at}geol.lu.se

Cu-Ni ore bodies from five Botswana mines were investigated, using a combination of X-ray diffraction, thermomagnetic measurements, Mössbauer spectroscopy, and Inductively Coupled Plasma-Atomic Absorption Electro-optic Spectroscopy (ICP-AES). The four dominant phases in the ores were found to be pyrrhotite, magnetite, pentlandite, and chalcopyrite. Mössbauer spectroscopy shows that pyrrhotite is the dominant Fe-bearing phase in the samples with 75% in B-S1, 84% in B-S2, and 80% in B-S4 and was selected for detailed characterization. Our results show that the three first techniques provide an excellent approach to estimate the degree of polytype intermixing in pyrrhotite. Accordingly, the ores can be grouped into three types: A hexagonal (or peak-type), with a 5c stacking arrangement; a monoclinic (or Weiss-type), with a 4c stacking arrangement; and a mixture of monoclinic and hexagonal types. A low-temperature study of the monoclinic form shows that the low-temperature magnetic transition close to 35 K has features similar to the Verwey transition in magnetite induced by Jahn-Teller distortion due to orbital order-disorder transition.

Key-words: pyrrhotite, Mössbauer spectroscopy, magnetism, Verwey transition, polytype.






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