Akaganeite is a tunnel-structured iron oxyhydroxide that frequently forms in chloride-rich environments. It typically appears as brownish, needle-like acicular crystal clusters or earthy crusts and is often found as a weathering product in iron deposits.
Is this akaganeite?
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 akaganeite with a known reference. Akaganeite sits at Mohs 3-4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: brownish yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: adamantine to metallic.
- 4Match the color rangeCompare against the expected color range: brown, rusty brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: massive, powdery.
Often confused with
Akaganeite 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. 3-4); luster reads adamantine to metallic on Akaganeite and imperfect adamantine metallic to dull earthy; silky when fibrous on Goethite.
How to tell apart: Lepidocrocite is the harder of the two (Mohs 5 vs. 3-4); streak differs, so Akaganeite leaves brownish yellow, Lepidocrocite leaves dull orange; luster reads adamantine to metallic on Akaganeite and submetallic on Lepidocrocite.
How to tell apart: Hematite is the harder of the two (Mohs 5-6 vs. 3-4); streak differs, so Akaganeite leaves brownish yellow, Hematite leaves cherry-red or reddish brown; luster reads adamantine to metallic on Akaganeite and metallic or submetallic to dull on Hematite.
Often found alongside akaganeite
Minerals reported to co-occur with akaganeite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Fe,Ni)₈(OH,O)₁₆Cl₁.₂₅·nH₂O
- Mohs hardness
- 3-4
- Density
- 3 g/cm³
- Colors
- BrownRusty brown
- Streak
- Brownish yellow
- Luster
- Adamantine to metallic
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Massive, powdery
- Cleavage
- Distinct on {110}
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- Apparently formed from pyrrhotite by ferric-sulfate-bearing solutions (Akagané mine, Japan); an important constituent in some soils, and certain geothermal brines and in sea-floor nodules
Where rockhounds find akaganeite
Classic worldwide localities
- Black Rock, Humboldt Co., Nevada, USA
- North of the Santa Niño mine, Santa Cruz Co., Arizona, USA
- Questa, Taos Co., New Mexico, USA
- Duluth Gabbro complex, near Hibbing, St. Louis Co., Minnesota, USA
- Akagané mine, Iwate Prefecture, Japan
- Found at the Las Animas mine, La Mur, Trincheras, Sonora, Mexico
Field-hunting tip
Where it forms: Apparently formed from pyrrhotite by ferric-sulfate-bearing solutions (Akagané mine, Japan); an important constituent in some soils, and certain geothermal brines and in sea-floor nodules. If you start seeing pyrrhotite, hibbingite in float, you are in the right ground. Typical habit: massive, powdery. Train your eye for that shape before scanning the outcrop.
Common questions
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Find akaganeite on the map
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