Ferrihydrite

The Fe analogue of akdalaite.

A poorly crystalline nanomineral built up of about 20% (FeO4) and 80% (FeO6) polyhedra. It forms by rapid oxidation and hydrolysis, and exists in varying degrees of structure disorder; the two extremes are the so-called 2-line and 6-line ferrihydrites (distinguished by their X-ray powder diffraction patterns). It is a metastable nanomineral precursor to other minerals, e.g. hematite, goethite. There are no known deposits of ferrihydrite before the Pleistocene.

Two structure models have been proposed: the "f-phase" model and the "akdalaite" model; the first one seems to be correct (Paktunc et al., 2013).

Some lichen (e.g., Pertusaria corallina, Stereocaulon vulcani) may induce ferrihydrite formation on lava flows and basalt- and gabbro- hosted olivine and also on augite (see Burford et al., 2003).

The iron bacteria, especially Gallionella ferruginea, Leptothrix ochracea and Toxothrix trichogenes create ferrihydrite as a waste product of their metabolism.