Excelsior Mine, Cerro de Pasco, Pasco province, Pasco department, Peru © Rob Lavinsky

Chemical Formula: Pb9As4S15
Locality: In the Excelsior mine, Cerro de Pasco, Peru, in large crystals.
Name Origin: For Louis Carly Graton (1880-1970), Professor of Economic Geology, Harvard University, Cambridge, Massachusetts, USA.

Gratonite is a lead-arsenic sulfosalt mineral, with the chemical composition Pb9As4S15. Gratonite was discovered in 1939 at the Excelsior Mine, Cerro de Pasco, Peru. It is named in honor of geologist L. C. Graton (1880–1970), who had a long-standing association with the Cerro de Pasco mines.

Physical Properties

Cleavage: None
Color: Lead gray, Dark lead gray.
Density: 6.22
Diaphaneity: Opaque
Fracture: Brittle – Generally displayed by glasses and most non-metallic minerals.
Hardness: 2.5 – Finger Nail
Luster: Metallic
Streak: black

Photo :

This sample of gratonite is displayed in the Smithsonian Museum of Natural History. The sample at left is about 3×5 cm and is from Cerro de Pasco, Junin, Peru.
Gratonite Locality: Excelsior Mine, Cerro de Pasco, Daniel Alcides Carrión Province, Pasco Department, Peru (Locality at Size: thumbnail, 2.2 x 1.2 x 1.2 cm © Rob Lavinsky / iRocks
Gratonite Locality: Excelsior Mine, Cerro de Pasco, Peru (Type Locality for Gratonite) Overall Size:    16x11x7 mm © AllMinerals/JohnBetts-FineMinerals