Alloclasite is a rare cobalt arsenic sulfide that closely resembles cobaltite and arsenopyrite in hand samples. It typically occurs as metallic, steel-gray masses or small prismatic crystals in cobalt-rich hydrothermal veins. Because of its arsenic content, it is a mineral intended for advanced collectors who can manage its toxicity safely.
Is this alloclasite?
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 alloclasite with a known reference. Alloclasite sits at Mohs 5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: nearly black.
- 3Read the lusterHold the specimen under a strong light. Luster: bright metallic.
- 4Match the color rangeCompare against the expected color range: steel gray.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: aggregates, prismatic, radial.
Often confused with
Alloclasite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Alloclasite leaves nearly black, Cobaltite leaves grayish black; luster reads bright metallic on Alloclasite and metallic on Cobaltite.
How to tell apart: Streak differs, so Alloclasite leaves nearly black, Arsenopyrite leaves black; luster reads bright metallic on Alloclasite and metallic on Arsenopyrite.
How to tell apart: Streak differs, so Alloclasite leaves nearly black, Glaucodot leaves black; luster reads bright metallic on Alloclasite and metallic on Glaucodot.
Often found alongside alloclasite
Minerals reported to co-occur with alloclasite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Co(AsS)
- Mohs hardness
- 5
- Density
- 5.91-5.95 g/cm³
- Colors
- Streak
- Nearly black
- Luster
- Bright metallic
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Aggregates, prismatic, radial
- Cleavage
- Perfect on {101}, distinct on {010}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In calcite or quartz veins of apparently low-temperature, late-stage hydrothermal origin
Where rockhounds find alloclasite
Classic worldwide localities
- Kibblehouse quarry, Perkiomenville, Montgomery Co., Pennsylvania, USA
- Oraviţa (Oravicza) (type locality), as in the Elizabeth mine, Romania
- Bieber, near Hanau, Hesse, Germany
- San Juan de Plan mine, Gistain Valley, Huesca, Spain
- Silverfields mine, Cobalt, and the Siscoe Metals mine, Miller Lake, Gowganda, Ontario, Canada
- Bou Azzer, Morocco
Field-hunting tip
Where it forms: In calcite or quartz veins of apparently low-temperature, late-stage hydrothermal origin. If you start seeing gold, silver, glaucodot in float, you are in the right ground. Typical habit: aggregates, prismatic, radial. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify alloclasite?+
Where is alloclasite found?+
Is alloclasite safe to handle?+
What rocks look like alloclasite?+
What minerals are found with alloclasite?+
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Find alloclasite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
