Bournonite - Encyclopedia

    Class : Sulphides and sulfosalts
    Subclass : Sulfosalts
    Crystal system : Orthorhombic
    Chemistry : PbCuSbS3
    Rarity : Quite common


Bournonite is a relatively common sulfo-antimonide in medium and low temperature hydrothermal veins. It frequently associated with galena, pyrite, sphalerite, tetrahedrite, chalcopyrite, fluorite and barite. It owes its name to the French crystallographer, Count Jacques Louis de Bournon who described the mineral. It is presented in flattened pseudo-tetragonal crystals, often implanted on the edge with striated prism faces, in short, tabular prismatic crystals with fluted faces up to 11 cm, or in compact masses sometimes grainy. The bournonite is especially famous for its characteristic cruciform twins in "cogwheel". It has an often bright metallic luster and a steel gray color tending toward black. Highly weatherable, it frequently gives rise to a procession of variously colored oxides of copper, lead and antimony. Bournonite has occasionally been used as copper , lead and sometimes antimony ore, it is no longer used today but its crystals are still appreciated by mineral collectors.

Bournonite from Loiras, Herault, France
Twinned bournonite from Quiruvilca Mine, Peru
Bournonite on quartz from Pachapaqui District, Peru
Bournonite from Les Malines, Gard, France

Bournonite in the World

Exceptional crystals come from English Cornwall (15 cm twinned crystals) and from Harz in Germany (15 cm tabular crystals on siderite). Superb centimetric crystals were also extracted from the Bolivian mines (Vibora : 15 cm crystals) and Peruvian (Oraya : 4 cm crystals).

Bournonite in France

In France, interesting groups of crystals were found at Pontgibaud (Puy-de-Dôme : 3 cm tabular crystals), at the Barlet fluorite mine (Langeac, Haute-Loire), at Les Maline Mine (Gard : 6 cm crystals), but especially at the Prunières mine (La Mure, Isère) which provided the most beautiful specimens : 5 cm tabular crystals implanted on siderite and tetrahedrite. The quarry of Loiras (Hérault) has when it provided crystals up to 2 cm on white dolomite.

Twinning

Very common on {110} this twin repeats to form crystals with cruciform facies called "cogwheels".

Fakes and scams

Some crystals, from Les Malines (Gard) especially, could be brushed with Dremel to improve the surface quality.



Hardness : 2.5 to 3
Density : 5.83
Fracture : Irregular to sub-conchoidal
Streak : Gray




TP : Opaque
RI : Not measurable
Birefringence : Not measurable
Optical character : None
Pleochroism : None
Fluorescence : None


Solubility : Nitric acid

Magnetism : Paramagnetic
Radioactivity : None