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Atacamite prisms from Chile
Category Halide mineral
(repeating unit)
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
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 [1][2][3][4]

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 1801.[1]

Atacamite is polymorphous with botallackite, clinoatacamite, and paratacamite.[1] 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 and as alteration of ancient bronze and copper artefacts.[2] It occurs in association with cuprite, brochantite, linarite, caledonite, malachite, chrysocolla and its polymorphs.[2]

Atacamite from Mt. Gunson, South Australia

It has been shown that atacamite is a component of the jaws of some Glycera species.[5]


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