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 | |
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![]() 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/cm3 |
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
edit- ^ 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.
- ^ a b Handbook of Mineralogy
- ^ a b c Mindat with locations
- ^ Webmineral data