In soils and sediments, Fe(III)-(oxyhydr)oxides are critical secondary minerals that control iron bioavailability and the cycling of coupled elements. The nanomineral ferrihydrite is a poorly crystalline and metastable initial form typically resulting from Fe(II) oxidation during meteoric recharge to stagnant pore water. Because it is metastable, ferrihydrite will spontaneously transform into more persistent bulk phases such as goethite. But this transformation is kinetically hindered, consistent with both 1) the extremely low aqueous solubility of Fe(III) restricting dissolution/reprecipitation, and 2) the highly activated solid-state diffusion of iron restricting topotactic transformation. Particle-based crystallization has been proposed but this cannot transform ferrihydrite without also recruiting one or both of these two pathways.<br/>Here we report a coupled experimental and computational investigation into the slow transformation of ferrihydrite to goethite resolved at the iron site occupancy level. Two-line ferrihydrite anoxically aged over two years was periodically characterized using in situ uXRD with detailed properties modeling, electron microscopy, and synchrotron Fe L-edge and O K-edge XAS/XMCD quantitatively analyzed using MRCI ab initio calculations. The data are compared to DFT calculations of the relative stabilities and activation energies for solid-state diffusion of iron within archetypal ferrihydrite structure models. Tetrahedral Fe(III) is clearly resolved in 2 d ferrihydrite, diminishing systematically with increasing octahedral Fe(III) as well as trace Fe(II) with a concomitant decrease in the net magnetic moment until the first appearance of goethite at ~ 100 d. DFT calculations reveal corresponding topotactic iron diffusion pathways with activation energies as low as 1 eV per unit cell. The findings are being assembled into transformation kinetics models that will help resolve the relative importance of dissolution/reprecipitation vs. solid-state pathways, to yield the first complete mechanism of this important Fe(III)-(oxyhydr)oxide mineral transformation.

Atomically-resolved spontaneous transformation of ferrihydrite to goethite

2019 Spring National Meeting

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