Atacamite

Atacamite
	240px Atacamite prisms from Chile
General
Category	Halide mineral
Formula; (repeating unit)	Cu2Cl(OH)3
Strunz classification	03.DA.10a
Crystal symmetry	Orthorhombic dipyramidal; H-M symbol: (2/m 2/m 2/m); Space group: Pmcn
Unit cell	a = 6.03 Å, b = 9.12 Å, 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
Crystal system	Orthorhombic
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

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.^[4]

References

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