Crookesite is an exceptionally rare copper-thallium-silver selenide mineral typically found as small, lead-gray metallic grains in hydrothermal deposits. Collectors prize it primarily for its historical significance in the discovery of the element thallium and its inclusion in rare-earth selenide assemblages. It is almost exclusively found in association with other selenide minerals and requires careful handling due to its toxic chemical composition.
Is this crookesite?
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 crookesite with a known reference. Crookesite sits at Mohs 2.5-3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: lead-gray.
- 3Read the lusterHold the specimen under a strong light. Luster: metallic.
- 4Match the color rangeCompare against the expected color range: blue gray, brown pink, gray.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: disseminated.
Often confused with
Crookesite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Crookesite leaves lead-gray, Eucairite leaves shining.
How to tell apart: Streak differs, so Crookesite leaves lead-gray, Berzelianite leaves shining.
How to tell apart: Streak differs, so Crookesite leaves lead-gray, Clausthalite leaves grayish black.
Often found alongside crookesite
Minerals reported to co-occur with crookesite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Cu₇TlSe₄
- Mohs hardness
- 2.5-3
- Density
- 6.90 g/cm³
- Streak
- Lead-gray
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Tetragonal
- Crystal habit
- Disseminated
- Cleavage
- Two at right angles, fairly well developed
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- Of hydrothermal origin, with other selenides
Where rockhounds find crookesite
Classic worldwide localities
- Skrikerum, near Tryserum, Kalmar, Sweden (type locality)
- Tilkerode, Harz Mountains, Germany
- Bukov, near Tisnova, and in the Petrovice uranium deposit, near Ždǎr, Czech Republic
- Pinky Fault uranium deposit, near Lake Athabasca, Saskatchewan, Canada
- Tuminico, Sierra de Cacho, La Rioja Province, Argentina
Field-hunting tip
Where it forms: Of hydrothermal origin, with other selenides. If you start seeing umangite, berzelianite, eucairite in float, you are in the right ground. Typical habit: disseminated. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify crookesite?+
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Find crookesite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
