The natural nanomineral ferrihydrite is an important component of many environmental and soil systems and has been implicated as the inorganic core of ferritin in biological systems. Knowledge of its basic structure, composition, and extent of structural disorder is essential for understanding its reactivity, stability, and magnetic behavior, as well as changes in these properties during aging. Here we investigate compositional, structural, and magnetic changes that occur upon aging of "2-line" ferrihydrite in the presence of adsorbed citrate at elevated temperature. Whereas aging under these conditions ultimately results in the formation of hematite, analysis of the atomic pair distribution function and complementary physicochemical and magnetic data indicate formation of an intermediate ferrihydrite phase of larger particle size with few defects, more structural relaxation and electron spin ordering, and pronounced ferrimagnetism relative to its disordered ferrihydrite precursor. Our results represent an important conceptual advance in understanding the nature of structural disorder in ferrihydrite and its relation to the magnetic structure and also serve to validate a controversial, recently proposed structural model for this phase. In addition, the pathway we identify for forming ferrimagnetic ferrihydrite potentially explains the magnetic enhancement that typically precedes formation of hematite in aerobic soil and weathering environments. Such magnetic enhancement has been attributed to the formation of poorly understood, nano-sized ferrimagnets from a ferrihydrite precursor. Whereas elevated temperatures drive the transformation on timescales feasible for laboratory studies, our results also suggest that ferrimagnetic ferrihydrite could form naturally at ambient temperature given sufficient time.crystal structure | disorder | nano-sized ferrimagnets | soil formation | strain T he structural and physical properties of ferrihydrite, an exclusively nano-sized ferric oxyhydroxide, are of importance in explaining its chemical reactivity and wide variety of occurrences. In both pristine and contaminated soils and sediments, ferrihydrite acts as a natural filter of inorganic contaminants through sorption reactions, thus affecting their transport and fate in the environment. Biomineralization of ferrihydrite as the inorganic iron core in ferritin-the protein mainly involved in iron storage and homeostasis in the human body-also occurs in a vast number of organisms (1). Bloom-forming marine diatoms, for example, use ferritin for enhanced iron storage (2), which suggests that ferrihydrite may also have underlying importance in primary productivity in the world's oceans.A well-known example of a nanomineral (3), ferrihydrite has no known crystalline counterpart formed in the laboratory or found in nature. As such, the basic crystal structure (4-7) and physical properties of ferrihydrite [e.g., density, composition (7, 8), and magnetic properties (9-14)] have remained controversial. A variety of structural models ha...
