Selective dissolution of Fe–Ti oxides — Extractable iron as a criterion for andic properties revisited

An important criterion used to determine andic properties in World Reference Base is Al + ½ Fe content (by ammonium oxalate) equal to 2% or more. Experiments have shown however that the acid–ammonium oxalate-extractable Fe (Feox) value often does not only express the amorphous or short-range-order (SRO) Fe-constituents, typical for andic properties, but is influenced by the dissolution of Fe in magnetite and maghemite lattices. This paper discusses the influence of Fe–Ti oxides in particular titanomagnetite, a common constituent of many volcanic rocks, on Feox values and proposes an alternative method for determination of SRO Fe-constituents. Mineralogical analyses of fresh dolerite, separated magnetic fractions and single titanomagnetite grains were carried out using scanning electron microscopy, electron microprobe, and Mössbauer spectroscopy. Iron was selectively extracted using acid–ammonium oxalate (Feox), Na dithionite–citrate–bicarbonate (Fed) and Tiron (Fetiron). The concentrations of Feox in all samples were higher than Fed and Feox/Fed ratios progressively decreased from the strongly magnetic fraction to the weakly magnetic and non-magnetic fractions, proving the influence of titanomagnetite on Feox. On the other hand, no notable difference was observed between Fetiron and Feox in the different fractions, indicating the very limited influence of titanomagnetite on Fe extraction using Tiron. Consequently the Tiron extraction method is being recommended as a better alternative to the oxalate method to quantify amorphous/SRO Fe-constituents in soils and regoliths containing magnetic minerals, and especially to identify andic properties in soils on volcanic ash.