Ferrihydrite is a poorly crystalline, naturally occurring iron oxyhydroxide found predominantly in oxidized iron deposits and modern soils. It usually appears as reddish-brown, earthy, or powdery coatings rather than distinct crystals, making it a challenge to identify without laboratory analysis. It is often a precursor to more stable iron minerals like goethite or hematite.
Is this ferrihydrite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch ferrihydrite with a known reference. Ferrihydrite sits at Mohs 2-3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Ferrihydrite leaves a reddish-brown streak.
- 3Read the lusterHold the specimen under a strong light. Ferrihydrite typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: reddish-brown, brown, dark-brown.
- 5Look at form & habitCrystal system: trigonal. Typical habit: fine-grained, earthy, botryoidal, or powdery.
Often confused with
Ferrihydrite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Iron Ore is the harder of the two (Mohs 5-6.5 vs. 2-3); streak differs, so Ferrihydrite leaves reddish-brown, Iron Ore leaves reddish-brown to black; luster reads dull on Ferrihydrite and metallic to submetallic on Iron Ore.
How to tell apart: Limonite is the harder of the two (Mohs 4-5.5 vs. 2-3); streak differs, so Ferrihydrite leaves reddish-brown, Limonite leaves yellowish-brown; luster reads dull on Ferrihydrite and submetallic to earthy on Limonite.
Often found alongside ferrihydrite
Minerals reported to co-occur with ferrihydrite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe₅HO₈·4H₂O
- Mohs hardness
- 2-3
- Density
- 3.8-4.0 g/cm³
- Streak
- Reddish-brown
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Fine-grained, Earthy, Botryoidal, Or Powdery
- Cleavage
- None
- Rarity
- Common
- Uses
- Collector
- Host rock
- Oxidized Zones of Hydrothermal Deposits, Sedimentary Iron Formations, And Soils
- Typical price
- $10-50 for small samples
Where rockhounds find ferrihydrite
Classic worldwide localities
- Germany
- United States
- United Kingdom
- Russia
- Brazil
Field-hunting tip
Look in oxidized zones of hydrothermal deposits, sedimentary iron formations, and soils country, the host setting where ferrihydrite typically forms. If you start seeing goethite, hematite, lepidocrocite in float, you are in the right ground. Field specimens usually show a fine-grained, earthy, botryoidal, or powdery habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find ferrihydrite on the map
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