Atacamite
Atacamite is a copper chloride hydroxide, hardness 3 to 3.5, named for the Atacama Desert. Its polymorphs, the Cornish connection at Botallack, and storage.
Atacamite is a copper chloride hydroxide, hardness 3 to 3.5, forming bright to dark emerald-green orthorhombic crystals up to 10 cm long. It is named for the Atacama Desert of northern Chile. The apple-green streak separates it from malachite, and it needs chloride to form at all.
In short
- Chloride is the point. Atacamite forms where copper oxidises in the presence of salt — arid and saline ground, marine spray, fumaroles, and seafloor sulphide deposits. It is a chemical signature of the environment, not just a copper mineral.
- It has three polymorphs with identical chemistry: botallackite, clinoatacamite and paratacamite. They cannot be told apart by eye, and one of them — botallackite — is named for a Cornish mine.
- The apple-green streak is the cheap test. Malachite's streak is a paler green and malachite fizzes in cold dilute acid; atacamite does not.
- It also forms as a corrosion product on bronze and copper objects of antiquity, which is why conservators of archaeological metalwork know the species as well as mineralogists do.
- Crystals are slender prisms elongated along [001] and striated, commonly tabular or pseudo-octahedral, and often twinned into contact and penetration groups.
| Species | Formula | System | How it differs | Where it is classic |
|---|---|---|---|---|
| Atacamite | Cu2Cl(OH)3 | Orthorhombic | The common polymorph; slender striated prisms | Atacama Desert, Chile |
| Botallackite | Cu2Cl(OH)3 | Monoclinic | Rare, and the least stable of the three | Botallack, St Just, Cornwall |
| Clinoatacamite | Cu2Cl(OH)3 | Monoclinic | Not separable by eye; requires analysis | Widespread, often misidentified as paratacamite |
| Paratacamite | Cu2(OH)3Cl, zinc-bearing | Rhombohedral | Not separable by eye | Chile, and as a corrosion product |
| Malachite | Cu2(CO3)(OH)2 | Monoclinic | Effervesces in cold dilute acid; paler green streak | Congo, Tsumeb, Bisbee |
| Dioptase | CuSiO3 H2O | Trigonal | Hardness 5; sharp rhombohedral crystals; no chloride | Tsumeb, Altyn-Tyube, Congo |
Why atacamite forms where it does
Copper carbonates form almost anywhere copper oxidises. Atacamite needs chloride as well, and that requirement determines its whole distribution. The Handbook of Mineralogy describes it as an oxidation product of other copper minerals, especially under arid, saline conditions; in fumarolic deposits; as a weathering product of sulphides in subsea black smoker deposits; and as an alteration of bronze and copper objects of antiquity.
Hence the geography. Northern Chile's copper deposits sit under one of the driest climates on earth with saline groundwater, and the species is named for the Atacama, from an occurrence in the desert that was never precisely located. At Chuquicamata it is abundant enough to be an ore mineral rather than a curiosity. The other classic settings follow the same logic: Vesuvius and Etna for the fumarolic route, and the Mid-Atlantic Ridge's TAG hydrothermal field for the seafloor one.
The Cornish occurrences are the coastal version of the same story. Cornwall's copper mines worked ground within reach of sea spray, and material is recorded from Botallack at St Just and from the Penberthy Croft mine at St Hilary, alongside Roughtongill in the Caldbeck Fells.
Botallackite, and the polymorph problem
Three minerals share atacamite's formula. Botallackite is monoclinic and takes its name from Botallack mine in St Just, one of the most famous of the Cornish coastal copper mines. Clinoatacamite is also monoclinic; paratacamite is rhombohedral and normally zinc-bearing. All are green, all form in the same environments, and none can be separated from atacamite by eye.
What this means for a collector is straightforward and slightly deflating. A green copper chloride labelled atacamite is a reasonable presumption, not a determination, unless the specimen has been analysed. A good deal of older material labelled paratacamite has since proved to be clinoatacamite, which was only recognised as a distinct species relatively recently, so the literature and old labels are out of step. Where a specimen has been analysed, that fact belongs on the label with the method and the date, and it materially increases what the specimen is worth to anybody who cares.
The reasonable approach is the one we recommend generally on identification: record what the label says, record what you can establish, and keep the two separate. Do not overwrite an old determination with a confident new one you cannot support.
Telling it from malachite, and other identification
Atacamite's green is a different green from malachite's — more emerald, less blue-green, and often much darker, running to blackish green. That impression is real but not evidence. Three tests are:
- Streak. Atacamite's is apple-green, distinctly brighter and yellower than malachite's paler green. Cheap, fast, and non-destructive if done on an inconspicuous edge.
- Acid. Malachite is a carbonate and effervesces vigorously in cold dilute hydrochloric acid. Atacamite does not. Decisive, but destructive — use the smallest drop on the back of the specimen, and not at all on a good piece.
- Habit. Malachite is overwhelmingly botryoidal or acicular in radiating sprays; atacamite's slender striated prisms and pseudo-octahedral crystals are quite unlike it. Where crystals are present, this settles it without touching the specimen.
Hardness is little help at 3 to 3.5 against malachite's 3.5 to 4. Density is more useful than it looks: atacamite is 3.75 against malachite's roughly 4.0, but that margin is too narrow for a displacement measurement on a matrix specimen.
Storage, and the one real hazard
Atacamite is chemically stable in ordinary conditions and does not need the sealed, desiccated treatment that the sulphides do. Two cautions apply.
First, keep it dry, not because it decays but because it is a chloride: moisture mobilises chloride, and a damp atacamite in a drawer can corrode anything metallic nearby, including specimen pins, mounts and any native metal specimens. Conservators of archaeological bronze deal with exactly this problem, where the same mineral is the visible symptom of active chloride corrosion in a metal artefact.
Second, it is soft. At hardness 3 to 3.5, with perfect cleavage on {010} and a brittle tenacity, the slender prisms damage easily. Individual boxes, and support under the matrix.
Toxicity is not a real concern here beyond the general rule — copper minerals are among the least hazardous things in a collection, as our page on toxic minerals sets out, and normal handwashing is enough. The species that share its localities are a different matter: Chilean and Cornish copper ground carries arsenates, and those take the arsenic rules.
Good crystallised atacamite, and particularly analysed Cornish material with a proper mine name, does not appear often. If it is what you want, the wanted list is the way to tell us. We hold no stock; what we can do is ask whether a piece has ever been analysed before it changes hands.