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 — 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 — that is 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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