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Dioptase (blue green), cerussite (light pink) and fornacite (green) from Renéville, Djoué, Brazzaville Region, Republic of Congo
Category Arsenate mineral
(repeating unit)
Strunz classification 7.FC.10
Dana classification
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group P21/c
Unit cell a = 8.101(2),
b = 5.893(11),
c = 17.547(9) [Å];
β = 110.00(4)°; Z = 4
Color Deep olive-green
Crystal habit Aggregates of steep pyramidal to bladed, rounded crystals
Fracture Irregular/uneven, conchoidal, sub-conchoidal
Tenacity Brittle
Mohs scale hardness 2 - 3
Luster Resinous, waxy, greasy
Streak Olive green
Diaphaneity Transparent
Density 6.27 g/cm3
Optical properties Biaxial (+)
Refractive index nα = 2.142 nγ = 2.242
Birefringence δ = 0.100
2V angle Large
References [1][2][3]

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.[2] 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, Congo Republic.[2]

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.[1]