Fornacite

(repeating unit)Pb₂Cu(CrO₄)(AsO₄)(OH)IMA symbolFor^[1]Strunz classification7.FC.10Dana classification43.4.3.2Crystal systemMonoclinicCrystal classPrismatic (2/m)
(same H-M symbol)Space groupP2₁/cUnit cella = 8.101(2),
b = 5.893(11),
c = 17.547(9) [Å];
β = 110.00(4)°; Z = 4IdentificationColorDeep olive-greenCrystal habitAggregates of steep pyramidal to bladed, rounded crystalsFractureIrregular/uneven, conchoidal, sub-conchoidalTenacityBrittleMohs scale hardness2–3LusterResinous, waxy, greasyStreakOlive greenDiaphaneityTransparentDensity6.27 g/cm³Optical propertiesBiaxial (+)Refractive indexn_α = 2.142 n_γ = 2.242Birefringenceδ = 0.1002V angleLargeReferences^[2][3][4]

Fornacite is a rare lead, copper chromate arsenate hydroxide mineral with the formula: Pb₂Cu(CrO₄)(AsO₄)(OH). It forms a series with the phosphate mineral vauquelinite.^[3] It forms variably green to yellow, translucent to transparent crystals in the monoclinic – prismatic crystal system. It has a Mohs hardness of 2.3 and a specific gravity of 6.27.

It was first described in 1915 and named after Lucien Lewis Forneau (1867–1930) the governor of the French Congo. Its type locality is in Reneville, Republic of Congo.^[3]

It occurs in the oxidized zone of ore deposits and is associated with dioptase, wulfenite, hemihedrite, phoenicochroite, duftite, mimetite, shattuckite, chrysocolla, hemimorphite, willemite and fluorite.^[2]