Cacotrichite is a delicate, fibrous variety of goethite that forms fine, hair-like tufts or needles. It is highly prized by collectors for its brilliant silky luster and golden-yellow color, though its fragile nature makes it difficult to collect and display. It is typically found in the oxidized zones of iron ore deposits and hydrothermal veins.
Is this cacotrichite?
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 cacotrichite with a known reference. Cacotrichite sits at Mohs 5-5.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: yellow-brown.
- 3Read the lusterHold the specimen under a strong light. Luster: silky.
- 4Match the color rangeCompare against the expected color range: yellow, golden-yellow, brown.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: acicular crystals, hair-like fibers, fibrous tufts.
Often confused with
Cacotrichite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Cacotrichite is noticeably harder (Mohs 5-5.5 vs. 3-3.5); streak differs, so Cacotrichite leaves yellow-brown, Millerite leaves greenish black; luster reads silky on Cacotrichite and metallic on Millerite.
How to tell apart: Cacotrichite is noticeably harder (Mohs 5-5.5 vs. 4); streak differs, so Cacotrichite leaves yellow-brown, Heazlewoodite leaves light bronze; luster reads silky on Cacotrichite and metallic on Heazlewoodite.
How to tell apart: Cacotrichite is noticeably harder (Mohs 5-5.5 vs. 2); streak differs, so Cacotrichite leaves yellow-brown, Melanterite leaves white; luster reads silky on Cacotrichite and vitreous on Melanterite.
Often found alongside cacotrichite
Minerals reported to co-occur with cacotrichite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- FeO(OH)
- Mohs hardness
- 5-5.5
- Density
- 3.3-4.3 g/cm³
- Colors
- Streak
- Yellow-brown
- Luster
- Silky
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Acicular crystals, hair-like fibers, fibrous tufts
- Cleavage
- Perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- oxidized hydrothermal zones, iron-rich deposits
Where rockhounds find cacotrichite
1 mapped spotsClassic worldwide localities
- Erzgebirge, Germany
- Cornwall, England
- Moravia, Czech Republic
- Arizona, USA
Field-hunting tip
Where it forms: oxidized hydrothermal zones, iron-rich deposits. If you start seeing goethite, limonite, quartz in float, you are in the right ground. Typical habit: acicular crystals, hair-like fibers, fibrous tufts. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah. Start trip planning there.
Common questions
How do you identify cacotrichite?+
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What rocks look like cacotrichite?+
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Find cacotrichite on the map
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