Francevillite - Encyclopedia

    Class : Phosphates, arsenates, vanadates
    Subclass : Uranyl vanadates
    Crystal system : Orthorhombic
    Chemistry : (Ba,Pb)(UO2)2(VO4)2 5H2O
    Rarity : Very rare


Francevillite is a vanadate of uranium, barium and lead, discovered in abundance in the oxidation zones of uranium deposits where it comes from the oxidation of uraninite. It forms a continuous series with curienite, a mineral in which lead completely replaces barium. The intermediate terms, rich in lead, see their color change to red-orange and olive green. It owes its name to its discovery location : the Mounana mine in Franceville (Gabon). Francevillite occurs in decimetric incrustations made up of aggregates of amber-yellow crystals, diamond-shaped, flattened according to {001} or bipyramidal with curved faces, reaching 8 mm. In certain samples francevillite appears in veinlets, coatings, arborescences or radiated aggregates in the form of spherules of almost centimeter size. Locally, it is an important uranium ore.

Main photo : Francevillite from Mounana Mine, Gabon © Stephan Wolfsried

Francevillite from Mounana Mine, Gabon © Eugene & Sharon Cisneros
Francevillite from Mounana Mine, Gabon © Paul de Bondt
Francevillite from Mounana Mine, Gabon © Stephan Wolfsried
Francevillite from Mounana Mine, Gabon © Cédric Lheur

Francevillite in the World

The best samples come from Mounana in Gabon, they are decimetric incrustations. Francevillite is also known in the USA at Mahoning (Pennsylvania) and at the Pandora mine (Utah), as well as in Germany (Clara Mine) and at Musonoi (Democratic Republic of Congo).

Francevillite in France

In France, francevillite was the main uranium ore in the St-Pierre-du-Cantal (Cantal) deposit, where it permeated the Oligocene sands and clays. Its composition is very close to the pure barytic pole. This deposit was exploited by SCUMRA between 1959 and June 1985. Francevillite is also reported at Echassières (Allier), Liauzun (Puy-de-Dôme), Gerardmer (Vosges), Rodez and Gage (Aveyron), as well as than at Cap Garonne (Var).

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.



Hardness : 3
Density : 4.55
Fracture : Undetermined
Streak : Light yellow


TP : Translucent
RI : 1.750 to 2.002
Birefringence : 0.195 to 0.217
Optical character : Biaxial -
Pleochroism : Visible
Fluorescence : None


Solubility : -


Magnetism : None
Radioactivity : Strong

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