Chalcoalumite is a rare secondary copper mineral that typically forms as delicate, sky-blue crusts or velvety coatings in oxidized ore deposits. It is most frequently found in the weathered zones of copper mines where it often accompanies other bright blue or green copper carbonates and sulfates.
Is this chalcoalumite?
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 chalcoalumite with a known reference. Chalcoalumite 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. Chalcoalumite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Chalcoalumite typically shows a pearly luster.
- 4Match the color rangeCompare against the expected color range: sky-blue, blue-green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: crusts, mammillary, or platy aggregates.
Often confused with
Chalcoalumite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Turquoise is the harder of the two (Mohs 5-6 vs. 2.5); luster reads pearly on Chalcoalumite and waxy on Turquoise.
How to tell apart: Luster reads pearly on Chalcoalumite and vitreous on Chrysocolla.
How to tell apart: Luster reads pearly on Chalcoalumite and vitreous on Allophane.
Often found alongside chalcoalumite
Minerals reported to co-occur with chalcoalumite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CuAl₄(SO₄)(OH)₁₂·3H₂O
- Mohs hardness
- 2.5
- Density
- 2.2-2.3 g/cm³
- Colors
- Streak
- White
- Luster
- Pearly
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Crusts, Mammillary, Or Platy Aggregates
- Cleavage
- Perfect On {001}
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- Oxidized Copper Deposits
- Typical price
- $15-80 for small cabinet specimens
Where rockhounds find chalcoalumite
Classic worldwide localities
- Bisbee, Arizona, USA
- Chuquicamata, Chile
- Tsumeb, Namibia
- Katanga, DR Congo
Field-hunting tip
Look in oxidized copper deposits country — that is the host setting where chalcoalumite typically forms. If you start seeing azurite, malachite, brochantite in float, you are in the right ground. Field specimens usually show a crusts, mammillary, or platy aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find chalcoalumite on the map
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