FE-SPECIATION IN KAOLINS - A DIFFUSE-REFLECTANCE STUDY

Diffuse reflectance spectra of kaolins have been recorded in samples f rom different environments. They show the systematic presence of Fe-ox ides, even in bleached kaolins, with no contribution from the Fe3+ ion s substituted in kaolinite. Second derivative spectra of various Fe-ph ases (hematite, goethite, lepidocrocite, maghemite, akaganeite, ferrih ydrite and Fe-polymer) may be differentiated by the position of a diag nostic band corresponding to the 2(6A1) --> 2(4T1(4G)) transition. The systematic comparison of diffuse reflectance spectra of unbleached an d bleached kaolins has demonstrated the differences between the Fe-oxi des occurring as coatings and as occluded phases. The features observe d in second derivative spectral curves are consistent with assignments of crystal field transitions to goethite, hematite, akaganeite, and a ged hydrous ferric oxides. The optical determination of the Fe-phases associated to kaolins assists in the interpretation of the formation c onditions of these minerals.