Atacamite
Atacamite is a copper chloride hydroxide mineral, Cu2Cl(OH)3, that forms a striking emerald to blackish-green secondary mineral in the oxidized zones of copper deposits. It takes its name from the Atacama Desert of Chile, where it was first described and where arid conditions allow chloride-bearing copper minerals to persist rather than being washed away. Atacamite is notable among copper minerals for containing structural chlorine, which ties its formation to saline or coastal environments.
Atacamite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | Cu2Cl(OH)3 |
| Mohs hardness | 3–3.5 |
| Crystal system | Orthorhombic |
| Luster | Vitreous to adamantine |
| Streak | Apple green |
| Cleavage | Perfect, one direction |
| Fracture | Conchoidal |
| Specific gravity | 3.75–3.77 |
| Transparency | Transparent to translucent |
| Common colors | Bright green, Dark green |
| Rarity | common |
Geology & background
Atacamite forms when copper sulfide ores oxidize in the presence of chloride-rich groundwater or saline soils, conditions especially well developed in hyperarid deserts where evaporation concentrates chlorides near the water table. Beyond its type locality in Chile's copper districts, notable occurrences include the Mount Gunson deposits of South Australia, ancient slag deposits in the Mediterranean, and copper mines in Namibia and Peru's coastal desert belt. It can also form as a corrosion product on ancient bronze artifacts and shipwreck copper, giving it relevance to archaeology as well as mineralogy.
Collectors value atacamite for its vivid green color and, less commonly, for slender prismatic crystals with a diamond-like luster, though most specimens are fibrous, granular, or botryoidal crusts. Because it is a chloride mineral, it can be more chemically reactive than typical copper carbonates like malachite, so specimens should be kept dry and away from acidic conditions to avoid slow deterioration. Fine crystallized atacamite from Chile and Australia is genuinely scarce compared to the more commonly seen massive material.
How to identify Atacamite
Atacamite ranges from bright emerald green to dark blackish-green, with a vitreous to adamantine luster on crystal faces and a pale green streak. It has a hardness of 3 to 3.5, perfect cleavage in one direction, and typically forms slender prismatic or acicular crystals, though botryoidal and granular masses are more common. It closely resembles malachite and brochantite in color, but atacamite lacks malachite's effervescence in dilute acid and its crystals tend to be more elongated and less banded than malachite's characteristic concentric growth.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Atacamite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Atacamite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Atacamite forms one of the green corrosion products found on ancient bronze statues and shipwreck artifacts exposed to seawater.
- It was first scientifically described from Chile's Atacama Desert, one of the driest places on Earth.
- Atacamite is dimorphous with paratacamite and botallackite, meaning all three share the identical chemical formula but crystallize in different atomic arrangements.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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