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 3.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Alloclasite leaves a black streak.
- 3Read the lusterHold the specimen under a strong light. Alloclasite typically shows a metallic luster.
- 4Match the color rangeCompare against the expected color range: white, steel-gray.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: prismatic crystals, massive, granular.
Often confused with
Alloclasite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Cobaltite is the harder of the two (Mohs 5.5 vs. 3.5); streak differs, so Alloclasite leaves black, Cobaltite leaves greyish-black.
How to tell apart: Arsenopyrite is the harder of the two (Mohs 5.5-6 vs. 3.5).
How to tell apart: Glaucodot is the harder of the two (Mohs 5 vs. 3.5).
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,Fe)AsS
- Mohs hardness
- 3.5
- Density
- 6.55 g/cm³
- Colors
- Streak
- Black
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Prismatic Crystals, Massive, Granular
- Cleavage
- Distinct in One Direction
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Hydrothermal Veins
- Typical price
- $20-150 for thumbnail specimens
Where rockhounds find alloclasite
Classic worldwide localities
- Oravita, Romania
- Bou Azzer, Morocco
- Siberia, Russia
- Ontario, Canada
Field-hunting tip
Look in hydrothermal veins country, the host setting where alloclasite typically forms. If you start seeing cobaltite, arsenopyrite, safflorite in float, you are in the right ground. Field specimens usually show a prismatic crystals, massive, granular habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find alloclasite on the map
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