To first order, reduced iron favors isotopically light iron and oxidized iron favors isotopically heavy iron. This effect has been studied in regards to the abiotic oxidation of Fe to Fe, which results in fractionation. The mineral ferrihydrite, which forms in acidic aquatic conditions, is precipitated via the oxidation of aqueous Fe to Fe. Precipitated ferrihydrite has been found to be enriched in the heavy isotopes by 0.45‰ per atomic mass unit with respect to the starting material. This indicates that heavier iron isotopes are preferentially precipitated as a result of oxidizing processes.
Theoretical calculations in combination with experimental data have also aimed to quantify the fractionation between Fe(III) and Fe(II) in HCl. Based on modeling, the fractionation factor between the two species is temperature dependent: