Évaluation du rôle du fer structural sur la cristallinité des kaolinites

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Abstract
This paper deals with 24 kaolinite samples whose content in Fe₂O₃ varies from zero up to about 2 %. Though iron can also be accomodated in ancillary minerals (such as titanium oxides and micas) contaminating the kaolinite, the selected samples proved to be representative of kaolinites with different contents in structural iron. The treatment of the ESR iron spectra of these kaolinites permits the quantitative identification of two types of iron sites, called here E iron and I iron. The content in E iron is not correlated to the degree of kaolinite crystallinity. In contrast, the content in I iron is directly related to the proportion of defects due to the displacement of the vacant site in the octahedral sheet. When occurring in kaolinite, such defects induce the formation of domains whose hydroxylic configuration becomes similar to that of dickite. In any kaolinite, the proportion of such domains can be identified by the quantitative treatment of features of the OH stretching vibration bands of its infrared spectrum. The results show that a multiple relation exists between the proportion of defects due to a vacancy displacement, the content in I iron and the proportion of domains showing a hydroxylic configuration similar to that of dickite. The most disordered kaolinites are characterized by an I iron content of about 1 % (expressed in Fe₂O₃) and a proportion of dickite-like domains of about 50 %.

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