Cornwallite is a secondary copper arsenate mineral typically forming vibrant green, botryoidal or crusty coatings on matrix. It is highly sought after by collectors for its resemblance to malachite, though it can be distinguished by its distinct habit and associated rare copper minerals. It is predominantly found in the weathered zones of copper-arsenic ore deposits.
Is this cornwallite?
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 cornwallite with a known reference. Cornwallite sits at Mohs 4.5, 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: light green, dark green, black green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: botryoidal, crusts, radial aggregates.
Often confused with
Cornwallite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads vitreous on Cornwallite and adamantine to vitreous; silky if fibrous; dull to earthy if massive on Malachite.
How to tell apart: Streak differs, so Cornwallite leaves pale green, Conichalcite leaves green.
Often found alongside cornwallite
Minerals reported to co-occur with cornwallite. 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
- 4.5
- Density
- 4.17 g/cm³
- Streak
- Pale green
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Botryoidal, crusts, radial aggregates
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A rare secondary mineral in oxidized copper deposits
Where rockhounds find cornwallite
Classic worldwide localities
- Mammoth and Centennial Eureka mines, Tintic district, Juab Co., and Gold Hill mine, Tooele Co., Utah, USA
- Majuba Hill mine, Antelope district, Pershing Co., Nevada, USA
- Cornwall, from Wheals Gorland and Unity, Gwennap, at Wheal Carpenter, Gwinear, England
- Potts Gill mine, Caldbeck Fells, Cumbria, England
- Sainte-Marie-aux-Mines, Haut-Rhin, Triembach-au-Val, Haut-Rhin, and at the Cap Garonne mine, near le Pradet, Var, France
- Clara Mine, near Oberwolfach, and at Neubulach, Black Forest, Germany
Field-hunting tip
Where it forms: A rare secondary mineral in oxidized copper deposits. If you start seeing olivenite, cornubite, arthurite in float, you are in the right ground. Typical habit: botryoidal, crusts, radial aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify cornwallite?+
Where is cornwallite found?+
Is cornwallite safe to handle?+
What rocks look like cornwallite?+
What minerals are found with cornwallite?+
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Find cornwallite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
