FRANCKEITE

    Class : Sulfides and sulfosalts
    Subclass : Sulfides
    Crystal system : Triclinic
    Chemistry : FePb5Sn3Sb2S14
    Rarity : Rare

Franckeite is a very rare lead and tin sulfide present in certain tin-bearing hydrothermal veins, mainly in Bolivia. It was named in honor of the two German mining engineers Carl and Ernest Francke, pioneers of Bolivian geology in the 19th century. Franckeite occurs in fine, lamellar crystals, more rarely elongated and grooved on {010}, often curved, but more commonly in masses, with a foliated or radiated appearance, in multicentimetric spherical aggregates, in rosettes or cauliflowers reaching 1 cm in diameter. Franckeite is lead gray to gray-black in color. It is a very minor ore of tin.

Main photo : Franckeite de San José Mine, Oruro, Cercado Province, Oruro, Bolivia © Oleg Lopatkin

Franckeite of San José Mine, Oruro, Cercado Province, Oruro, Bolivia © Brian Kosnar
Franckeéite of San José Mine, Oruro, Cercado Province, Oruro, Bolivia © Gianfranco Ciccolini
Franckeite from San José Mine, Oruro, Cercado Province, Oruro, Bolivia © E. & S. Cisneros
Franckeite from Siglo Veinte Mine, Llallagua, Potosí, Bolivia © Tony Peterson

Franckeite in the World

Franckeite is best known in the tin-bearing veins of Bolivia, where most of the well-individualized crystals come from, some exceeding 6 cm and grouped in tufted aggregates (Poopo, juanuni, Chocaya, Llallagua...). Franckéite is also known elsewhere, notably at Renison Bell (Tasmania, Australia) and in California, at Kalkar, near Santa Cruz, and at Thompson, near Darwin.

Franckeite in France

In France, franckéite forms microscopic areas in the wolframite vein of Vens-Haut (Puy-de-Dôme).

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 2.5 to 3
Density : 5.90
Fracture : Irregular
Streak : Grey-Black

TP : Opaque
RI : -
Birefringence : -
Optical character : -
Pleochroism : None
Fluorescence : None

Solubility : Nitric acid


Magnetism : None
Radioactivity : None

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