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 — 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 — 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 — that is 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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