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 in the World
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