The Effects of Magnetite on Oxalate‐ and Dithionite‐Extractable Iron

Abstract

AbstractSix soils from a chronosequence were chosen to test the effects of magnetite on using free Fe to estimate relative ages of soils. The acid‐ammonium oxalate and dithionite‐citrate‐bicarbonate methods were used to extract the free Fe from the samples. Iron values from these two extractions were compared for samples from which the magnetite was removed, from which the magnetite was not removed, and from the magnetite fraction itself. Visual examinations were also made of the untreated and treated magnetite grains. The oxalate solution releases Fe from magnetite in proportion to the amount of magnetite in the soil. The dithionite solution does not release Fe from magnetite, but releases Fe from hematite coatings and replacements on the magnetite grains. Large or varying magnetite concentrations can influence the amount of Fe released by either of these methods, and cause errors in estimations of relative age. Magnetite should be removed from the samples prior to treatment by oxalate extraction. Magnetite should be left in the samples to be treated by the dithionite extraction, but the concentration of magnetite in the soils should be noted. In order to get the best possible estimation of relative age using free Fe concentrations, the ratio of magnetite‐free oxalate to dithionite‐extractable Fe should be used.