Formula: Fe3+10O14(OH)2
Simple oxide
Specific gravity: 3.96
Streak: Yellow-brown
Colour: Dark brown, yellow-brown
Solubility: Highly soluble in ammonium oxalate (Dana)

Hydrothermal environments

Ferrihydrite forms by rapid oxidation and hydrolysis, and exists in varying degrees of structure disorder. It is a metastable precursor to other minerals such as hematite and goethite. Some lichen may induce ferrihydrite formation on lava flows and basalt-hosted and gabbro-hosted olivine and also on augite. The iron bacteria produce ferrihydrite as a waste product of their metabolism.
Ferrihydrite is found in cold-water springs, acid mine effluents, warm subsea springs, warm water seabeds, thermal springs and thermal water in brooks, rivers and lakes. Associated minerals include scorodite, goethite, lepidocrocite, hematite and manganese oxides (AM 60.485-486, Mindat, Dana, HOM).


At the Niagara Basin of Lake Ontario, Canada, ferrihydrite was found in an iron and phosphorus rich crusty red layer in sediment cores. Immediately overlying the 0.5 cm-thick red layer is a thin manganese-rich black layer. It is suggested that the layer formed around 65 years ago at the sediment-water interface during a brief period (a few years) of very low sedimentation, permitting crystallisation of brown ferric hydroxides to red ferrihydrite. The red layer invariably overlies 3 to 4 cm of very light grey Fe3+ and phosphorus-deficient mud (CM 23.103-110).

There are two co-type localities, the Belousovsky mine, Glubokoye, and the Ridder-Sokolnoe mine, Ridder, both in East Kazakhstan.

At the Franklin Seamount, Woodlark Basin, Papua New Guinea, large deposits of iron-silicon-manganese oxyhydroxide, intimately associated with active warm springs, cover the flanks and caldera of the seamount. The deposits are dominated by poorly crystalline ferrihydrite. The ferrihydrite is very stable, not transforming to hematite until 570oC (CM 37.973-990).

At the Peña del Hierro mine, Rio Tinto Mines, Minas de Riotinto, Huelva, Andalusia, Spain, samples were collected that contained sulphate minerals including gypsum, jarosite and copiapite, and iron hydroxide bearing minerals including goethite and ferrihydrite. Minor quartz occurred in some samples (AM 99.1199-1205).

At the Clark Fork River Superfund Complex, Montana, USA, ferrihydrite and a vernadite-like mineral were found in samples collected from the riverbeds and floodplains of the river draining the largest mining-contaminated site in the United States. The contaminant heavy metals, arsenic, copper, lead and zinc, are always associated with these minerals. In several field specimens, the ferrihydrite and vernadite-like mineral are intimately mixed on the nanoscale, but they also occur separately (AM 90.718-724).

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