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. Streak: yellowish brown.
- 3Read the lusterHold the specimen under a strong light. Luster: dull.
- 4Match the color rangeCompare against the expected color range: yellow brown, dark brown.
- 5Look at form & habitCrystal system: hexagonal. Typical habit: aggregates, microscopic crystals.
Often confused with
Ferrihydrite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Goethite is the harder of the two (Mohs 5-5.5 vs. 2-3); streak differs, so Ferrihydrite leaves yellowish brown, Goethite leaves brownish yellow, yellow-orange, ocher-yellow; luster reads dull on Ferrihydrite and imperfect adamantine metallic to dull earthy; silky when fibrous on Goethite.
How to tell apart: Hematite is the harder of the two (Mohs 5-6 vs. 2-3); streak differs, so Ferrihydrite leaves yellowish brown, Hematite leaves cherry-red or reddish brown; luster reads dull on Ferrihydrite and metallic or submetallic to dull on Hematite.
How to tell apart: Limonite is the harder of the two (Mohs 4-5.5 vs. 2-3); streak differs, so Ferrihydrite leaves yellowish 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
- Fe₅O₇(OH)
- Mohs hardness
- 2-3
- Density
- 3.8 g/cm³
- Colors
- Streak
- Yellowish brown
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Hexagonal
- Crystal habit
- Aggregates, microscopic crystals
- Cleavage
- None
- Rarity
- Common
- Uses
- Collector
- Host rock
- Widespread in the soluble fraction of soils and weathered rock
Where rockhounds find ferrihydrite
Classic worldwide localities
- Mineral Hill, Delaware Co., Pennsylvania, USA
- Hot Springs, Garland Co., Arkansas, USA
- Niagara basin of Lake Ontario, Arkansas, USA
- Ridder (Leninogorsk) and Belousovsk Pb–Zn mines, Rudnyi Altai, Kazakhstan
- Joensuu, Pohjiskarjala, Finland
- Killiechronan, southwest of Salen, Isle of Mull, Scotland
Field-hunting tip
Where it forms: Widespread in the soluble fraction of soils and weathered rock. If you start seeing goethite, lepidocrocite, hematite in float, you are in the right ground. Typical habit: aggregates, microscopic crystals. Train your eye for that shape before scanning the outcrop.
More ferrihydrite photos
Common questions
How do you identify ferrihydrite?+
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What rocks look like ferrihydrite?+
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Find ferrihydrite on the map
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