FTIR Reflectance vs. EPR Studies of Structural Iron in Kaolinites

Thierry Delineau1, Thierry Allard2, Jean-Pierre Muller2, 3, Odile Barres1, Jacques Yvon1 and Jean-Maurice Cases1
1 Laboratoire Environnement et Minéralurige, UA 235 CNRS, ENSG-INPL, B.P.40, 54501 Vandoeuvre Cédex, France
2 Laboratoire de Minéralogie et Cristallographie, Universités Paris VI et VII, 4, Place Jussieu, 75252 Paris Cedex 05, France
3 O.R.S.T.O.M., Départment T.O.A., URIG, 213, rue Lafayette, 75480 Paris Cedex 10, France

Abstract: The substitution of Fe3+ in the kaolinite structure is studied by EPR spectrometry and by FTIR spectrometry on a large set of kaolins from different origins (sedimentary and primary ores, soil kaolins). The IR bands at 3598 and 875 cm−1, observed in the literature only in the case of disordered kaolins or in Fe-rich environments (synthetic, lateritic), are revealed by high-resolution IR analysis, whatever the origin and the total Fe content of the samples. The EPR bands corresponding to Fe3+ substituted in sites II of the octahedral sheet increase when the IR absorbance near 3600 cm−1 increases. Two IR absorption bands near 4465 cm−1 and 7025 cm−1 are observed for the first time, both in transmission and diffuse reflectance on all samples. These bands are assigned to the combination of the 3598 and 875 cm−1 bands and to the first harmonic of the band at 3598 cm−1, respectively. The area of the band at 4465 cm−1 in diffuse reflectance is quantitatively correlated to the abundance of Fe3+ located in centers II as measured by ESR. This directly confirms the assignment of the two IR bands at 3598 and 875 cm−1 to OH stretching and deformation vibration bands in octahedral FE3+ environment in the kaolinite structure, respectively. Effects due to the size of particles and to the main kaolins impurities on the near infrared spectra, are also discussed.

Key Words: EPR • Fe • Kaolinite • Mid and near infrared • Substitution

Clays and Clay Minerals; June 1994 v. 42; no. 3; p. 308-320; DOI: 10.1346/CCMN.1994.0420309
© 1994, The Clay Minerals Society
Clay Minerals Society (www.clays.org)