Guilleminite is a rare secondary uranium mineral typically found as tiny, bright yellow to orange platy crystals. It is primarily recovered from the oxidized zones of uranium deposits where selenium is present. Due to its radioactive and toxic nature, it is strictly for advanced mineral collectors who practice careful storage and handling.
Is this guilleminite?
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 guilleminite with a known reference. Guilleminite sits at Mohs 2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: greenish yellow, yellow.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: platy crystals, tabular aggregates.
Often confused with
Guilleminite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Guilleminite leaves yellow, Derriksite leaves pale green; luster reads vitreous on Guilleminite and adamantine on Derriksite.
How to tell apart: Marthozite is the harder of the two (Mohs 6 vs. 2); streak differs, so Guilleminite leaves yellow, Marthozite leaves pale yellow.
Often found alongside guilleminite
Minerals reported to co-occur with guilleminite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ba(UO₂)₃(SeO₃)₂O₂·3H₂O
- Mohs hardness
- 2
- Density
- 4.88 g/cm³
- Colors
- Streak
- Yellow
- Luster
- Vitreous
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy crystals, tabular aggregates
- Cleavage
- Perfect on {100}; good on {010}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In the oxidized portions of a Cu–Co sulfide deposit (Musonoi mine, Congo)
Where rockhounds find guilleminite
Classic worldwide localities
- Musonoi Cu–Co mine, near Kolwezi, and at Shinkolobwe, Katanga Province, Congo (Shaba Province, Zaire)
Field-hunting tip
Where it forms: In the oxidized portions of a Cu–Co sulfide deposit (Musonoi mine, Congo). If you start seeing malachite, cuprosklodowskite, kasolite in float, you are in the right ground. Typical habit: platy crystals, tabular aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify guilleminite?+
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Is guilleminite safe to handle?+
What rocks look like guilleminite?+
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Find guilleminite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
