Fibroferrite is a hydrated iron sulfate that typically forms delicate, silky, fibrous masses or radiating acicular sprays. It is a secondary mineral found in the oxidation zones of iron-rich sulfide ore bodies, often occurring as an efflorescence in arid mining districts.
Is this fibroferrite?
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 fibroferrite with a known reference. Fibroferrite sits at Mohs ~2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: pearly, silky in aggregates.
- 4Match the color rangeCompare against the expected color range: green gray, yellow green, white, light green, light yellow.
- 5Look at form & habitCrystal system: trigonal. Typical habit: fibrous, massive, radial.
Often confused with
Fibroferrite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Fibroferrite leaves white, Copiapite leaves pale yellow.
How to tell apart: Fibroferrite is noticeably harder (Mohs ~2.5 vs. 1.5); luster reads pearly, silky in aggregates on Fibroferrite and vitreous on Halotrichite.
How to tell apart: Luster reads pearly, silky in aggregates on Fibroferrite and vitreous on Melanterite.
Often found alongside fibroferrite
Minerals reported to co-occur with fibroferrite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe³⁺(SO₄)(OH)·5H₂O
- Mohs hardness
- ~2.5
- Density
- 1.84-2.09 g/cm³
- Colors
- Streak
- White
- Luster
- Pearly, silky in aggregates
- Transparency
- Translucent
- Crystal system
- Trigonal
- Crystal habit
- Fibrous, massive, radial
- Cleavage
- On {001}, perfect
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- An uncommon dehydration product of melanterite in oxidized portions of rocks and sediments containing iron sulfide, most stable in arid regions; rarely a volcanic sublimate
Where rockhounds find fibroferrite
Classic worldwide localities
- Borate, about 10 km northeast of Yermo, Calico Hills, San Bernardino Co., at Island Mountain, Trinity Co., and in the Redington mine, Knoxville, Napa Co., California, USA
- Shattuck mine, Bisbee, Cochise Co., Arizona, USA
- Cordero mine, Poalit district, Humboldt Co., in the Commonwealth mine, Galena district, Washoe Co., Nevada, USA
- Dexter No. 7 mine, Calf Mesa, San Rafael district, Emery Co., Utah, USA
- Tierra Amarilla, near Coquimbó, Atacama, Chile
- Quetena, west of Calama, and at Chuquicamata, Antofagasta, Chile
Field-hunting tip
Where it forms: An uncommon dehydration product of melanterite in oxidized portions of rocks and sediments containing iron sulfide, most stable in arid regions; rarely a volcanic sublimate. If you start seeing melanterite, amarantite, hohmannite in float, you are in the right ground. Typical habit: fibrous, massive, radial. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify fibroferrite?+
Where is fibroferrite found?+
What rocks look like fibroferrite?+
What minerals are found with fibroferrite?+
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Find fibroferrite on the map
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