We show that by reacting ferrihydrite (FeH) with Fe (II) ions and subsequently increasing the pH, magnetite is formed through a multi-step nucleation process mediated by monodisperse FeH–Fe (II) primary particles. The interaction of these primary particles with a transient green rust phase leads to the formation of smaller secondary particles which form the feedstock for magnetite formation. Surprisingly, the presence of a polypeptide additive prevents the formation of green rust as an Fe (II) -rich intermediate phase, and leads to the formation of amorphous aggregates of FeH–Fe (II) particles which subsequently transform into the final magnetite nanocrystals. The observation of multiple transitions and the involvement of disordered precursor phases in this bioinspired crystallization route is important for our understanding of the nucleation of magnetite in geological and biological environments, and may lead to new approaches in the sustainable synthesis of this technologically important mineral.
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