Antlerite

Antlerite is a greenish hydrous copper sulfate mineral, with the formula Cu₃(SO₄)(OH)₄. It occurs in tabular, acicular, or fibrous crystals with a vitreous luster. Originally believed to be a rare mineral, antlerite was found to be the primary ore of the oxidised zones in several copper mines across the world, including the Chuquicamata mine in Chile, and the Antler mine in Arizona, US from which it takes its name. It is chemically and optically similar in many respects to other copper minerals such as malachite and brochantite, though it can be distinguished from the former by a lack of effervescence in hydrochloric acid.

Antlerite
A vein of acicular, green antlerite crystals from Chuquicamata, Chile (dimensions: 3.5 x 3.4 x 1.8 cm)
General
Category	Sulfate minerals
Formula (repeating unit)	Cu3(SO4)(OH)4
Strunz classification	7.BB.15
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma
Identification
Color	Bright green, through darker greens to black
Crystal habit	tabular and acicular or fibrous. Also found as reniform, massive or granular specimens
Cleavage	Perfect on [010]
Fracture	Uneven
Mohs scale hardness	3 to 3.5
Luster	Vitreous
Streak	Pale green
Diaphaneity	Translucent
Specific gravity	3.9
Optical properties	Biaxial (+)
Refractive index	nα = 1.726 nβ = 1.738 nγ = 1.789
Pleochroism	x: yellowish green y: bluish green z: bluish green
References	[1][2]

Antlerite is a common corrosion product on bronze sculptures located in urban areas, where atmospheric sulfur dioxide (a common pollutant) is present. Antlerite forms mainly in sheltered areas where weathering is low, which permits accumulation of copper ions and enhancement in the acidity of water films.[3] In exposed areas, the main corrosion product is brochantite.