Atacamite

Atacamite is a copper halide mineral: a copper(II) chloride hydroxide with formula Cu₂Cl(OH)₃. It was first described for deposits in the Atacama Desert of Chile in 1802 by Dmitri de Gallitzin.^[2] The Atacama Desert is also the namesake of the mineral.

Atacamite
Atacamite from Mt. Gunson mines, South Australia
General
Category	Halide mineral
Formula	Cu2Cl(OH)3
IMA symbol	Ata[1]
Strunz classification	3.DA.10a
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma
Unit cell	a = 6.03, b = 9.12 c = 6.865 [Å]; Z = 4
Identification
Color	Bright green, dark emerald-green to blackish green
Crystal habit	Slender prismatic crystals, fibrous, granular to compact, massive
Twinning	Contact and penetration with complex twinned groupings
Cleavage	Perfect on {010}, fair on {101}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	3–3.5
Luster	Adamantine to vitreous
Streak	Apple green
Diaphaneity	Transparent to translucent
Specific gravity	3.745–3.776
Optical properties	Biaxial (−)
Refractive index	nα = 1.831 nβ = 1.861 nγ = 1.880
Birefringence	δ = 0.049
Pleochroism	X = pale green; Y = yellow-green; Z = grass-green
2V angle	Calculated: 74°
Dispersion	r < v, strong
References	[2][3][4][5]

Occurrence

Atacamite is polymorphous with botallackite, clinoatacamite, and paratacamite.^[2] Atacamite is a comparatively rare mineral, formed from primary copper minerals in the oxidation or weathering zone of arid climates. It has also been reported as a volcanic sublimate from fumarole deposits, as sulfide alteration products in black smokers.^[3] The mineral has also been found naturally on oxidized copper deposits in Chile, China, Russia, Czech Republic, Arizona, and Australia.^[6] It occurs in association with cuprite, brochantite, linarite, caledonite, malachite, chrysocolla and its polymorphs.^[3]

Synthetic Occurrence

Atacamite has been discovered in the patina of the Statue of Liberty, and as alteration of ancient bronze and copper artifacts. The bronze of the Antikythera mechanism had turned to atacamite under the sea.^[7]

The mineral has been found as a pigment in sculpture, manuscripts, maps, and frescoes discovered in Eurasia, Russia, and Persia.^[6]

Biomineral

Atacamite occurs as a biomineral in the jaws of bloodworms.^[8][9]

• 
  Atacamite prisms from Chile
• 
  Atacamite from Chile displayed in the Harvard Museum of Natural History
• 
  Atacamite from Mt. Gunson, South Australia