Transformations induced in ferrihydrite by oven‐drying

Synthetic ferrihydrite preparations are metastable and on oven‐drying are shown to undergo significant transformation to more stable crystalline products the composition of which depends on the properties of the initial ferrihydrite. For a poorly ordered sample produced by rapid neutralization of a ferric nitrate solution, the dominant product was well‐crystalline hematite (α‐Fe2O3), the amount depending on the temperature at which drying was accomplished. Freeze‐drying prevented alteration of the ferrihydrite to more crystalline forms and, compared with any of the oven dried samples, minimized the change in rate of dissolution in acid ammonium oxalate solution. For a more ordered ferrihydrite produced by slow neutralization, oven‐drying caused the formation of some goethite (α‐FeOOH) with little or no hematite. The different transformation pathway for the more ordered ferrihydrite is attributed to 1) formation of larger polymers by slow growth during neutralization, resulting in a limited supply of the very small polymers which transform readily to hematite on heating, and 2) the probability of concomitant growth of goethite nuclei within the ferrihydrite allowing preferred transformation to goethite during oven‐drying.