THE MINERAL POLYBASITE

Chemistry: (Ag, Cu)16Sb2S11, Silver Copper Antimony Sulfide
Class: Sulfides
Subclass: Sulfosalts
Uses: An ore of silver and as mineral specimens.
Specimens

Polybasite is a somewhat uncommon silver bearing mineral. Although not a well known ore of silver, it is never-the-less a locally important ore in some mines. It forms interesting crystals that have a pseudohexagonal outline with rhombohedral striations. Although its symmetry is monoclinic, the nearly hexagonal crystals and striations suggest that it has a higher temperature phase that is hexagonal or trigonal.

Polybasite is in what is called a solid solution series with the mineral pearceite, (Ag, Cu)16As2S11. The two minerals can substitute the antimony for the arsenic within their structures. However, pearceite is far more rare than polybasite possibly indication that antimony is more stable in this structure. The solid solution series is similar to the one for pyrargyrite-proustite. Two other silver sulfosalts.

Polybasite is difficult to distinguish from the minerals hematite, an oxide, and another silver antimony sulfide, stephanite. Hematite forms metallic steel gray platy crystals but has a blood-red streak and is considerably harder. Stephanite lacks the rhombic striations (on the pinacoidal faces), the red flashes and the good cleavage of polybasite.

PHYSICAL CHARACTERISTICS:

Color is an iron black to a "cherry" red in thin slices with slight flashes of red possibly seen on some crystal surfaces.
Luster is metallic.
Transparency: Crystals are opaque to translucent in thin slices or at crystal edges.
Crystal System: Monoclinic; 2/m
Crystal Habits include pseudohexagonal plates or tablets, also massive.
Cleavage is perfect in one direction (basal).
Fracture is uneven.
Hardness is 2 - 3
Specific Gravity is approximately 6.1 - 6.3 (heavy even for metallic minerals)
Streak is a black to reddish-black.
Other Characteristics: Partially malleable, crystals will show a rhombic striation pattern at times and a dark coating can often form after prolonged exposure to light (can be removed by ultrasonic treatment).
Associated Minerals include silver, quartz, galena, proustite, pyrargyrite, stephanite, tetrahedrite , acanthite and other silver sulfide minerals.
Notable Occurrences include Las Chiapas, Guanajuato and Arizpe, Sonora, Mexico; Saxony, Germany; Colorado and Nevada, USA; Atacama, Chile; Bolivia; Australia and Sardinia.
Best Field Indicators are crystal habit, density, softness, cleavage, association with other silver sulfosalts and color.