Vauquelinite is a complex mineral with the formula CuPb₂(CrO₄)(PO₄)(OH) making it a combined chromate and phosphate of copper and lead. It forms a series with the arsenate mineral fornacite.^[2]

Vauquelinite
General
Category	Phosphate mineral
Formula (repeating unit)	CuPb2(CrO4)(PO4)(OH)
IMA symbol	Vql[1]
Strunz classification	7.FC.05
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/n
Unit cell	a = 13.754(5) Å, b = 5.806(6) Å c = 9.563(3) Å; β = 94.55°; Z = 4
Identification
Color	Olive-green, apple-green, brown to nearly black
Crystal habit	Irregular wedge shaped crystals, mammillary, reniform masses
Twinning	Present on {102}
Cleavage	Indistinct
Fracture	Irregular
Tenacity	Brittle
Mohs scale hardness	2.5 – 3
Luster	Adamantine, resinous
Streak	Greenish or brownish
Diaphaneity	Translucent
Specific gravity	6.16
Optical properties	Biaxial (−)
Refractive index	nα = 2.110 nβ = 2.220 nγ = 2.220
Birefringence	δ = 0.110
Pleochroism	X = Light green, Y = Light brown, Z = Light brown
2V angle	Near zero
References	[2][3][4]

It was first described in 1818 in the Beryozovskoye deposit, Urals, Russia, and named for Louis Vauquelin (1763–1829), a French chemist.^[2] It occurs in oxidized hydrothermal ore deposits and is associated with crocoite, pyromorphite, mimetite, cerussite, beudantite and duftite at the type locality in Russia.^[4]