Acanthite is the low-temperature stable form of silver sulfide and is the most important silver ore mineral. It often appears as dark, sectile masses or in distinctive pseudocubic crystals after argentite, which is unstable above 173°C.
Is this acanthite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch acanthite with a known reference. Acanthite sits at Mohs 2-2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: black.
- 3Read the lusterHold the specimen under a strong light. Luster: metallic.
- 4Match the color rangeCompare against the expected color range: lead-gray, black, gray, iron black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: arborescent, blocky, skeletal.
Often confused with
Acanthite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside acanthite
Minerals reported to co-occur with acanthite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ag₂S
- Mohs hardness
- 2-2.5
- Density
- 7.20-7.22 g/cm³
- Streak
- Black
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Arborescent, blocky, skeletal
- Cleavage
- Indistinct
- Rarity
- Common
- Uses
- Collector, Ore
- Host rock
- A common silver species in moderately low-temperature hydrothermal sulfide veins, and in zones of secondary enrichment
Where rockhounds find acanthite
1 mapped spotsClassic worldwide localities
- Butte, Silver Bow Co., Montana, USA
- Tonopah, Nye Co., and the Comstock Lode, Virginia City, Storey Co., Nevada, USA
- Jáchymov (Joachimsthal), Czech Republic (type locality)
- Freiberg, Schneeberg, Annaberg, and Marienberg, Saxony, Germany
- St. Andreasberg, Harz Mountains, Germany
- Large paramorphs from Arizpe, Sonora, Mexico
Field-hunting tip
Where it forms: A common silver species in moderately low-temperature hydrothermal sulfide veins, and in zones of secondary enrichment. If you start seeing silver, pyrargyrite, proustite in float, you are in the right ground. Typical habit: arborescent, blocky, skeletal. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Colorado. Start trip planning there.
Common questions
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Find acanthite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
