A method is proposed to measure the absolute concentration of paramagnetic
Fe3+ ions in kaolinite from various geochemical environments using powder X
-band electron paramagnetic resonance (EPR) data. An Fe3+-doped corundum sa
mple is used as a concentration standard. The Fe3+ signal is calibrated by
calculating the powder EPR spectra of Fe3+ ions in corundum and low-defect
kaolinite. The paramagnetic Fe3+ concentration in other samples is obtained
by an extrapolation procedure. This study provides a direct assessment of
the iron distribution between isolated structural Fe3+ ions and other iron
species, such as Fe3+ concentrated phases and Fe2+ ions. The concentration
of isolated structural Fe3+ ranges between 200-3000 ppm and represents less
than half of the total iron within kaolinite crystals.