A new model for transformation of ferrihydrite to hematite in soils and sediments

Ferrihydrite is among the most common precursors for iron-oxide formation in soils and sediments. Previous studies proposed a three-stage process for ferrihydrite transformation to hematite via an intermediate ferrimagnetic phase, which provides the basis for interpreting iron-oxide formation and its paleoenvironmental significance. From our new results, we identified two extra stages in this transformation, which fundamentally change the conventional model. The revised five-stage model is: (I) magnetically ordered (or core-shell structured) superparamagnetic (SP) ferrihydrite formation, (II) rapid hematite formation from SP ferrihydrite, (III) relative hematite stabilization, (IV) maghemite nanoparticle neoformation, and (V) completion of the transformation from maghemite to hematite. Unlike the previous model, in which pedogenic maghemite and hematite were formed mainly in stages I and II, we conclude that they are formed dominantly in stages IV and V, respectively. Thus, our work provides a new framework for interpreting pedogenic iron oxides and their paleoclimatic implications in natural environments.