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. Crookesite leaves a lead-gray streak.
- 3Read the lusterHold the specimen under a strong light. Crookesite typically shows a metallic luster.
- 4Match the color rangeCompare against the expected color range: lead-gray, bluish-white.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: massive, granular, or compact grains.
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 black.
How to tell apart: Streak differs, so Crookesite leaves lead-gray, Berzelianite leaves black.
How to tell apart: Streak differs, so Crookesite leaves lead-gray, Clausthalite leaves gray-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
- (Cu,Tl,Ag)₂Se
- Mohs hardness
- 2.5-3
- Density
- 6.9 g/cm³
- Colors
- Streak
- Lead-gray
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Tetragonal
- Crystal habit
- Massive, Granular, Or Compact Grains
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Scientific Research
- Host rock
- Hydrothermal Selenide Deposits
- Typical price
- $50-300 per small specimen
Where rockhounds find crookesite
Classic worldwide localities
- Skrikerum, Sweden
- Tumiñico, Argentina
- Siekierki, Poland
Field-hunting tip
Look in hydrothermal selenide deposits country, the host setting where crookesite typically forms. If you start seeing eucairite, berzelianite, clausthalite in float, you are in the right ground. Field specimens usually show a massive, granular, or compact grains habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find crookesite on the map
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