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Fornacite

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Rare lead, copper chromate arsenate hydroxide mineral
Fornacite
Dioptase (blue green), cerussite (light pink) and fornacite (green) from Renéville, Djoué, Brazzaville Region, Republic of Congo
General
CategoryArsenate mineral
Formula
(repeating unit)
Pb2Cu(CrO4)(AsO4)(OH)
IMA symbolFor
Strunz classification7.FC.10
Dana classification43.4.3.2
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/c
Unit cella = 8.101(2),
b = 5.893(11),
c = 17.547(9) ;
β = 110.00(4)°; Z = 4
Identification
ColorDeep olive-green
Crystal habitAggregates of steep pyramidal to bladed, rounded crystals
FractureIrregular/uneven, conchoidal, sub-conchoidal
TenacityBrittle
Mohs scale hardness2–3
LusterResinous, waxy, greasy
StreakOlive green
DiaphaneityTransparent
Density6.27 g/cm
Optical propertiesBiaxial (+)
Refractive indexnα = 2.142 nγ = 2.242
Birefringenceδ = 0.100
2V angleLarge
References

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. 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.

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.

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


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