Brochantite

Brochantite is a sulfate mineral, one of a number of cupric sulfates. Its chemical formula is Cu₄SO₄(OH)₆.[2][3][4] Formed in arid climates or in rapidly oxidizing copper sulfide deposits, it was named by Armand Lévy for his fellow Frenchman, geologist and mineralogist A. J. M. Brochant de Villiers.[4]

Brochantite
General
Category	Sulfate minerals
Formula (repeating unit)	Cu4SO4(OH)6
IMA symbol	Bct[1]
Strunz classification	7.BB.25
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/a
Identification
Color	Green, emerald green, or black
Crystal habit	Prismatic crystals; acicular needle-like crystals; druse
Cleavage	Perfect [100]
Fracture	Conchoidal
Tenacity	brittle
Mohs scale hardness	3.5 - 4.0
Luster	Vitreous to pearly
Streak	Pale green
Diaphaneity	Transparent to translucent
Specific gravity	3.97
Optical properties	Biaxial (-)
Refractive index	nα = 1.728 nβ = 1.771 nγ = 1.800
Birefringence	δ = 0.072
Pleochroism	Weak
2V angle	measured: 72°
References	[2][3][4]

Crystals of brochantite can range from emerald green to black-green to blue-green, and can be acicular or prismatic. Brochantite is often associated with minerals such as malachite, azurite, and chrysocolla, and may form pseudomorphs with these minerals.

The mineral is found in a number of locations around the world, notably the southwestern United States (especially Arizona), Serifos in Greece and Chile.

Brochantite is a common corrosion product on bronze sculptures located in urban areas, where atmospheric sulfur dioxide (a common pollutant) is present. Brochantitie forms mainly in exposed areas where weathering prevents accumulation copper ions and enhancement in the acidity of water films.[5] In sheltered areas, the main corrosion product is antlerite.