Acanthite is the principal silver-bearing sulfide and the most economically significant ore of silver. It is the low-temperature monoclinic polymorph of Ag2S; the high-temperature isometric polymorph (argentite) inverts to acanthite below 173°C, so essentially all natural Ag2S at room temperature is acanthite.
Properties
- Formula: Ag2S
- Crystal system: Monoclinic
- Hardness: 2 – 2.5
- Color: Black to lead-gray, tarnishing dull black
- Streak: Black, shining
- Luster: Metallic
- Cleavage: Indistinct
- Density: 7.2 – 7.4 g/cm³
Occurrence
Classic silver-mining districts: Guanajuato and Zacatecas (Mexico), Comstock Lode (Nevada), Kongsberg (Norway), Freiberg (Germany). In China, occurs as a minor accessory in some Pb-Zn-Ag deposits but rarely in collector quality.
Identification
Distinguished from galena (cubic cleavage, harder, lower density-relative-to-Ag) by lack of cleavage, sectile behavior (cuts like lead with a knife), and by association with native silver wires. Streak shines black on porcelain.
Collector Notes
Often intergrown with native silver wires — those specimens are among the most prized of all silver minerals. Mexican specimens dominate the modern market.
Found at these Localities
- Imiter Mine (伊米泰尔银矿)
- Kongsberg (康斯堡银矿)
- Cobalt, Ontario (安大略科博尔特银矿)
- Chañarcillo (查尼亚西略)
- Ying Silver Mine (银矿(豫西银多金属矿带))
