Compreignacite - Encyclopedia

    Class : Oxides and hydroxides
    Subclass : Uranyl hydroxides
    Crystal system : Orthorhombic
    Chemistry : K2(UO2)6O4(OH)6 8H2O
    Rarity : Very rare

Compreignacite was discovered in 1957 in the Margnac uranium deposit, in the town of Compreignac (Haute-Vienne, France) which gave it its name, then it was described a few years later. It constitutes one of these yellow products, gummites, which result from the transformation of uraninite when it oxidizes under the action of water and air, forming yellowish crusts on the periphery of the nodules of pitchblende. Compreignacite forms yellow crystals measuring a few tenths of a millimeter, with a parallelepiped appearance, sometimes twinned. It is the potassium analogue of billietite with which it is isostructural. It is usually found associated with uranotile, billietite and becquerelite.

Main photo : Coffinite from © L

Compreignacite in the World

Compreignacite is a very rare mineral which is only described in a handful of deposits in the world including that of White Canyon (Utah) of the Palermo Mine (New Hampshire), of Jáchymov (Czech Republic), of Lichtenberg (Germany) and the Eureka mine (Spain).

Right photo : Compreignacite and gypsum from Jáchymov, Czech Republic © Stephan Wolfsried

Compreignacite in France

In France, compreignacite is of course present in Margnac (Haute-Vienne) its type locality, but also in Rabejac and Mas d'Alary (Hérault).


twin is known on (110).

Fakes and treatments

No fakes recorded for this mineral species.

Hardness : Indeterminate
Density : 5.103
Fracture : Indeterminate
Streak : Light yellow

TP : Translucent to transparent
RI : 1.789 to 1.802
Birefringence : 0.013
Optical character : Biaxial -
Pleochroism : Strong
Fluorescence : Green

Solubility : -

Magnetism : None
Radioactivity : Very strong