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 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.^[7][8]

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