Aqueous formation and manipulation of the iron-oxo Keggin ion

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Science  20 Mar 2015:
Vol. 347, Issue 6228, pp. 1359-1362
DOI: 10.1126/science.aaa4620

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From iron clusters to iron mineral

Growing a mineral out of solution, either in the lab or in nature, requires the assembly of atoms or clusters of ions. The structure of some common iron oxides hints that tiny iron-oxygen clusters may serve as mineral building blocks, but isolating these often unstable clusters is challenging. Sadeghi et al. not only isolated but were able to control the growth and dissolution of an iron-oxygen cluster that is a likely precursor to the most common iron oxide mineral, ferrihydrite.

Science, this issue p. 1359


There is emerging evidence that growth of synthetic and natural phases occurs by the aggregation of prenucleation clusters, rather than classical atom-by-atom growth. Ferrihydrite, an iron oxyhydroxide mineral, is the common form of Fe3+ in soils and is also in the ferritin protein. We isolated a 10 angstrom discrete iron-oxo cluster (known as the Keggin ion, Fe13) that has the same structural features as ferrihydrite. The stabilization and manipulation of this highly reactive polyanion in water is controlled exclusively by its counterions. Upon dissolution of Fe13 in water with precipitation of its protecting Bi3+-counterions, it rapidly aggregates to ~22 angstrom spherical ferrihydrite nanoparticles. Fe13 may therefore also be a prenucleation cluster for ferrihydrite formation in natural systems, including by microbial and cellular processes.

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