Naturally occurring iron-(hydr)oxide particles have a strong influence over nutrient and contaminant transport in Earth's aqueous surface environments. Most phases have stoichiometrically balanced chemical formulas such as FeOOH and Fe2O3, but ferrihydrite—an abundant yet metastable, poorly crystalline precursor to more thermodynamically stable phases—is not defined by a simple ordering. Furthermore, all natural iron-(hydr)oxides undoubtedly contain numerous internal and adsorbed impurities that have varying effects on reactivity and stability. To understand how these impurities get incorporated into growing particles, Bazilevskaya et al. used a combined experimental and computational approach to track coprecipitation of ferrihydrite with varying amounts of Al3+. When Al3+ was present at low levels, it was incorporated into the ferrihydrite structure; at high Al3+ levels, in contrast, two distinct Al- and Fe-rich phases formed. Computations suggest that when Al-rich ferrihydrite transforms into Al-rich FeOOH over time, Al clusters within the new phase are energetically more favorable than isolated substitutions of Al3+ for Fe3+. In a complementary study, Hansel et al. documented the inhibitory effect of Al-substitution on ferrihydrite transformation. Ferrihydrite was more strongly inhibited from transforming into other, more thermodynamically stable phases when Al was adsorbed on the surface than when it was incorporated into the structure.
Geochim. Cosmochim. Acta 75, 10.1016/j.gca.2011.05.041; 10.1016/j.gca.2011.05.033 (2011).