Cornubite is a rare copper arsenate mineral often found as small, crusty, or botryoidal growths in the oxidized zones of ore deposits. It is visually similar to its dimorph, cornwallite, and collectors should look for its distinctive green, fibrous, or radial habits typically found alongside other secondary copper minerals.
Is this cornubite?
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 cornubite with a known reference. Cornubite sits at Mohs ~4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: pale green.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: green, apple green, dark green.
- 5Look at form & habitCrystal system: triclinic. Typical habit: botryoidal, crusts, radial aggregates, fibrous.
Often confused with
Cornubite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Cornubite is noticeably harder (Mohs ~4 vs. 3); streak differs, so Cornubite leaves pale green, Olivenite leaves olive-green to brown; luster reads vitreous on Cornubite and adamantine to vitreous, pearly to silky if fibrous on Olivenite.
How to tell apart: Luster reads vitreous on Cornubite and adamantine to vitreous; silky if fibrous; dull to earthy if massive on Malachite.
Often found alongside cornubite
Minerals reported to co-occur with cornubite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Cu₅(AsO₄)₂(OH)₄
- Mohs hardness
- ~4
- Density
- 4.64 g/cm³
- Streak
- Pale green
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Triclinic
- Crystal habit
- Botryoidal, crusts, radial aggregates, fibrous
- Cleavage
- Two observed, both ⊥ {211}, intersecting at about 70°
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- A rare secondary mineral in oxidized copper deposits
Where rockhounds find cornubite
Classic worldwide localities
- Centennial Eureka mine, Tintic district, Juab Co., and Gold Hill, Tooele Co., Utah, USA
- Majuba Hill mine, Antelope district, Pershing Co., Nevada, USA
- Humboldt and Marshall mines, Santa Cruz Co., Arizona, USA
- Sylvanite, Hidalgo Co., and in the Buckhorn mine, Red Cloud district, Lincoln Co., New Mexico, USA
- Cornwall, at Wheal Carpenter, Gwinear, England
- Wheals Gorland, Unity, and Muttrell, Gwennap, England
Field-hunting tip
Where it forms: A rare secondary mineral in oxidized copper deposits. If you start seeing cornwallite, chalcophyllite, olivenite in float, you are in the right ground. Typical habit: botryoidal, crusts, radial aggregates, fibrous. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify cornubite?+
Where is cornubite found?+
Is cornubite safe to handle?+
What rocks look like cornubite?+
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Find cornubite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
