Chalcothallite is an extremely rare copper-thallium sulfide/selenide mineral that is primarily known from the unique alkaline rocks of the Ilímaussaq complex. Collectors typically find it in small, opaque, dark metallic grains or thin lamellar masses associated with other rare thallium species.
Is this chalcothallite?
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 chalcothallite with a known reference. Chalcothallite sits at Mohs 2.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Chalcothallite leaves a black streak.
- 3Read the lusterHold the specimen under a strong light. Chalcothallite typically shows a metallic luster.
- 4Match the color rangeCompare against the expected color range: dark gray, black.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: tabular crystals, massive, lamellar.
Often confused with
Chalcothallite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside chalcothallite
Minerals reported to co-occur with chalcothallite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Cu₆Tl₂SbS₄Se₃
- Mohs hardness
- 2.5
- Density
- 6.08 g/cm³
- Streak
- Black
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Tetragonal
- Crystal habit
- Tabular Crystals, Massive, Lamellar
- Cleavage
- Perfect On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Alkaline Igneous Rocks
- Typical price
- $100-500 thumbnail size
Where rockhounds find chalcothallite
Classic worldwide localities
- Ilímaussaq complex, Greenland
- Taimyr Peninsula, Russia
Field-hunting tip
Look in alkaline igneous rocks country — that is the host setting where chalcothallite typically forms. If you start seeing galena, chalcocite, cuprite in float, you are in the right ground. Field specimens usually show a tabular crystals, massive, lamellar habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify chalcothallite?+
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Find chalcothallite on the map
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