Atacamite - Encyclopedia

Class : Halides
Subclass : Oxychlorides
Crystal system : Orthorhombic
Chemistry : Cu₂Cl(OH)₃
Rarity : Uncommon

Atacamite is an alteration mineral of copper deposits occurring in arid and saline conditions. It has three polymorphs : botallackite, paratacamite and clinoatacamite. It owes its name to its province of discovery : Atacama in Chile. It occurs in elongated prismatic crystals, striated along their length, in crystalline aggregates sometimes united in rosettes, and in fibrous masses, sometimes powdery. Atacamite is transparent to translucent, vivid, emerald green to blackish green, sometimes light green. Its formation in an arid and saline climate makes it a relatively widespread alteration mineral of copper by the sea : it is the secondary mineral that corrodes, for example, the brass parts of the Statue of Liberty in New York. It was once mined locally as copper ore, today its samples are exclusively intended for the collection.

Atacamite from Lily Mine, Pisco Umay, Ica, Peru

Atacamite in the World

The main deposits are those of the Atacama Desert (Chile), which provided the finest known specimens : centimetric lamellar crystals (up to 10 cm) assembled in rosettes ; atacamite was once mined there as copper ore. The Australian deposits of Burra and Moonta provide magnificent prismatic crystals of 8 cm ; other less spectacular crystals have been discovered in Mexican deposits (Boleo, Baja California).

Atacamite in France

In France, atacamite is described as microcrystals at Cap Garonne (Var).

Twinning and special forms

Rare and complex twins, difficult to observe on {110} ; most common on (544).

Fakes and scams

No fake recorded for this species.

Hardness : 3 to 3.5
Density : 3.745 to 3.776
Fracture : Conchoidal
Streak : Apple green

TP : Translucent to transparent
IR : 1.831 to 1.880
Birefringence : 0.049
Optical character : Biaxial -
Pleochroism : Weak
Fluorescence : None

Solubility : soluble in acids

Magnetism : None
Radioactivity : None

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