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Mineralogical Magazine; June 2002; v. 66; no. 3; p. 421-430; DOI: 10.1180/0026461026630038
© 2002 Mineralogical Society of Great Britain and Ireland
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Nature and mechanism of ilmenite alteration: a Mössbauer and X-ray diffraction study of oxidized ilmenite from the Beja-Acebuches Ophiolite Complex (SE Portugal)

J. C. Waerenborgh1,*, J. Figueiras2, A. Mateus2 and M. Gonçalves2

1 Departamento de Química, Instituto Tecnológico e Nuclear, P-2686-953 Sacavém, Portugal
2 Departamento de Geologia and CREMINER, Faculdade de Ciências da Universidade de Lisboa, P-1749-016 Lisbon, Portugal

* E-mail: jcarlos{at}itn1.itn.pt

Ilmenites from the least-altered rocks of the Beja-Acebuches Ophiolite Complex (SE Portugal), with low Ti values and excess Fe, despite rare optical evidence of hematite exsolution, were studied by 57Fe Mössbauer spectroscopy and X-ray diffraction. According to single-crystal XRD the sequence of alternate layers characteristic of the ideal ilmenite structure is preserved, the excess Fe being accommodated in the Ti layers. No superparamagnetic oxides were detected by 57Fe Mössbauer spectroscopy. The typical spectra of bulk {alpha}Fe2O3 and of Fe3+-containing ilmenite, in the paramagnetic state above 49 K and magnetically ordered at 6 K, are observed. The average degree of oxidation of the ilmenites, estimated from the chemical analysis assuming ideally stoichiometric full cation site occupancies, is also confirmed by 57Fe Mössbauer data. Since our crystal chemistry study gave no evidence of crypto-exsolution textures within the ilmenite with the observed compositions, fast cooling from magmatic temperatures and decomposition of ilmenite in supergene conditions is suggested.

KEYWORDS: ilmenite, Fe-Ti oxides, crystal chemistry, 57Fe Mössbauer spectroscopy, single-crystal X-ray diffraction




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