Iron in kaolinite with special reference to kaolinite from tropical soils

Abstract

Abstract The location of iron in five deferrated soil kaolinites and several reference samples has been investigated using chemical analyses, acid dissolution and electron paramagnetic resonance spectroscopy. The iron of most of the kaolinites can be divided into a 'structural iron' content, measured by the amount of iron, dissolved at the same rate as aluminium by acid attack, and a 'resistant iron' content, which is concentrated with the residues. Integration of the g eff ~4 resonance bands of the electron paramagnetic resonance (EPR) spectra allows a semiquantitative estimate of the 'structural iron' in the samples. A multiple relationship seems to exist between the crystallinity index, the 'structural iron' content and the shape and intensity of both g eff ~4 and g eff ~2 EPR signals. The tropical soil kaolinites are a peculiar category with a high content of iron (>2% structural Fe 2 O 3 ) fire-clay kaolinite, low crystallinity and a specific EPR spectrum exhibiting strong g eff ~4 resonance bands and no or weak g eff ~2 sharp doublet.