THE MINERAL GRATONITE

Chemistry: Pb9As4S15, Lead Arsenic Sulfide
Class: Sulfides
Subclass: Sulfosalts
Uses: Mineral specimens and as a very minor ore of lead.
Specimens

Gratonite is a rare lead sulfide mineral. It forms nice crystals that can be arranged into rosette-like clusters with projecting rhombohedral terminations. Clusters can be attractive and good specimens command large prices, mostly because of the rarity of the mineral. Gratonite is a sulfosalt, a segment of sulfides where the arsenic acts more like a metal than a non-metal and occupies a position where it is bonded to sulfurs.

PHYSICAL CHARACTERISTICS:

Color is a dark lead gray.
Luster is metallic.
Transparency: Crystals are opaque.
Crystal System: Trigonal; 3
Crystal Habits include rhombohedral as well as hexagonal prismatic crystals capped by three rhombohedral faces. Clusters can be found in rosette-like aggregates, also massive.
Cleavage: None.
Fracture: Subconchoidal.
Hardness is 2.5
Specific Gravity is 6.2 (heavier than average for metallic minerals)
Streak is gray.
Notable Occurrences are limited to the Excelsior Mine, Cerro de Pasco, Peru.
Best Field Indicators are crystal habit, locality, color and density.