FORNACITE

    Class : Sulfates, chromates, molybdates
    Subclass : Chromates
    Crystal system : Monoclinic
    Chemistry : Pb2Cu(AsO4)(CrO4)(OH)
    Rarity : Very rare

Fornacite is a rare chromate and arsenate of lead and copper whose genesis is conditioned by an arid climate. It is found associated with wulfenite, mimetite, sometimes dioptase, in the oxidation zones of lead, zinc and copper deposits. It was named in honor of Lucien Louis Fourneau, colonial governor of the French Congo where the mineral was discovered. Dark green in color, fornacite forms small prismatic crystals ending in pointed pyramids, it can also be lamellar to acicular. The crystals are usually grouped together.

Main photo : Fornacite de Renéville, Kindanba District, Democratic Republic of Congo © Andreas Schloth

Fornacite from Alice Mine, Goodsprings, Nevada, USA © Pierre Clolus
Fornacite from Singer Mine, Goodsprings, Nevada, USA © Andreas Schloth
Fornacite from Mammoth-St-Anthony Mine, Arizona, USA © Robert O. Meyer
Fornacite from Bluttenberg, Ste-Marie-aux-Mines, Haut-Rhin, France © T. Brunsperger

Fornacite in the World

The most beautiful fornacite crystals come from the copper deposit of Renéville (Congo), and the Argent mine near Ogies (South Africa), where individuals measuring 6 mm were extracted. Fornacite is relatively common in American deposits, particularly those in Arizona (Bisbee, Hayden, Tiger, etc.). It is also reported in Iran, and in several mines in Western Australia.

Fornacite in France

In France, fornacite has been reported in Bluttenberg near Ste-Marie-aux-Mines (Haut-Rhin).

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 2 to 3
Density : 6.27
Fracture : Irregular to conchoidal
Streak : Olive green

TP : Transparent
RI : 2.142 to 2.242
Birefringence : 0.100
Optical character : Biaxial +
Pleochroism : None
Fluorescence : None

Solubility : Hydrochloric acid


Magnetism : None
Radioactivity : None