Fornacite

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.

Fornacite
Dioptase (blue green), cerussite (light pink) and fornacite (green) from Renéville, Djoué, Brazzaville Region, Republic of Congo
General
Category	Arsenate mineral
Formula (repeating unit)	Pb2Cu(CrO4)(AsO4)(OH)
IMA symbol	For^[1]
Strunz classification	7.FC.10
Dana classification	43.4.3.2
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
Identification
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/cm^3
Optical properties	Biaxial (+)
Refractive index	nα = 2.142 nγ = 2.242
Birefringence	δ = 0.100
2V angle	Large
References	^[2]^[3]^[4]

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]

References

1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
2. Handbook of Mineralogy
3. Mindat with locations
4. Webmineral data