FERRIHYDRITE

'Ferrihydrite' is an iron oxyhydroxide with nominal formula 5Fe₂O₃·9H₂O. Although it is an accepted mineral there is much controversy about its crystal structure and crystal chemistry. It forms by rapid precipitation when reduced-iron-bearing ground water (or hydrothermal water) encounters oxygenated water.
The iron oxyhydroxide core of the ferritin protein is a biotic form of ferrihydrite. Most iron in biota, from bacteria to humans, is stored in this form.
Ferrihydrite is a common mining waste product resulting from iron removal in producing zinc and copper, for example. It is also produced in heavy metal removal technologies for purifying water.
Ferrihydrite is believed to be the main near-surface diagenetic iron oxyhydroxide in aquatic sediments however this is disputed.^[1] It is also believed to form biotically on the cell walls of bacteria in aquatic environments however this is also disputed.^[2]

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References

1. C. van der Zee, D. Roberts, D.G. Rancourt, C.P. Slomp. Nanogoethite is the dominant reactive oxyhydroxide phase in lake and marine sediments. ''Geology'' 31 (2003) 993-996.
2. D.G. Rancourt, P.-J. Thibault, D. Mavrocordatos, G. Lamarche. Hydrous ferric oxide precipitation in the presence of nonmetabolizing bacteria: Constraints on the mechanism of a biotic effect. ''Geochimica et Cosmochimica Acta'' 69 (2005) 553-577.