GRATONITE

    Class : Sulfides and sulfosalts
    Subclass : Sulfosalts
    Crystal system : Trigonal
    Chemistry : Pb9As4S15
    Rarity : Very rare

Gratonite is a very rare sulphoarsenide, present in certain hydrothermal deposits of lead, zinc and copper (veins, sulfidic masses, cover deposits, etc...), along with galena, sphalerite, chalcopyrite and arsenic minerals (enargite, tennantite). The presence of gratonite in the ore induces the formation of lead arsenates (particularly mimetite) in outcrops. It was named in honor of Louis Caryl Graton, professor of metallogeny at Harvard University. Gratonite forms elongated prismatic crystals up to 5 mm grouped in radiated aggregates up to 4 cm, and small lead-gray masses, opaque with a metallic luster.

Main photo : Gratonite from Excelsior Mine, Cerro de Pasco, Pasco province, Pasco, Peru © Rob Lavinsky

Gratonite from Excelsior Mine, Cerro de Pasco, Pasco province, Pasco, Peru © Rob Lavinsky
Gratonite from Excelsior Mine, Cerro de Pasco, Pasco province, Pasco, Peru © Rob Lavinsky
Gratonite from Excelsior Mine, Cerro de Pasco, Pasco, Peru © Gianfranco Ciccolini
Gratonite from Excelsior Mine, Cerro de Pasco, Pasco, Peru © Gianfranco Ciccolini

Gratonite in the World

The world's finest gratonite crystals come from the Excelsior mine near Cerro de Pasco (Peru) and measure 5 mm. The cracks cutting the polymetallic sulfidic masses of Rio Tinto (Huelva, Spain) also contain small gratonite crystals. This extremely rare mineral is also known in Tsumeb (Namibia), Pinar Del Rio (Cuba), and in a few other sites around the world.

Gratonite in France

Gratonite is not present in the French underground.

Twinning

No twins known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 2.5
Density : 6.22
Fracture : Irregular to conchoidal
Streak : Black

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

Solubility : -

Magnetism : NoneRadioactivity : None