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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)
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).
Localities
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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