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. Guilleminite leaves a yellow streak.
- 3Read the lusterHold the specimen under a strong light. Guilleminite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: orange, 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 — 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 3 vs. 2); streak differs — 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(OH)₂·5H₂O
- Mohs hardness
- 2
- Density
- 4.43 g/cm³
- Streak
- Yellow
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy Crystals, Tabular Aggregates
- Cleavage
- Perfect On {010}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Oxidized Zones of Uranium-selenium-bearing Hydrothermal Deposits
- Typical price
- $100-500 thumbnail
Where rockhounds find guilleminite
Classic worldwide localities
- Musonoi Mine, Democratic Republic of the Congo
- Shinkolobwe, Democratic Republic of the Congo
Field-hunting tip
Look in oxidized zones of uranium-selenium-bearing hydrothermal deposits country — that is the host setting where guilleminite typically forms. If you start seeing marthozite, derriksite, cuprosklodowskite in float, you are in the right ground. Field specimens usually show a platy crystals, tabular aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find guilleminite on the map
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