Marécottite is a rare uranyl sulfate mineral found primarily in the French-Swiss border region. It typically forms small, vibrant yellow to orange-yellow tabular crystals in oxidized zones of uranium deposits. Collectors should treat it with extreme caution due to its significant radioactive content.
Is this marécottite?
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 marécottite with a known reference. Marécottite sits at Mohs 3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: colorless.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: yellow orange.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: platy, rosette, triangular crystals.
Often confused with
Marécottite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Marécottite leaves colorless, Autunite leaves pale yellow.
How to tell apart: Streak differs, so Marécottite leaves colorless, Meta-autunite leaves yellow; luster reads vitreous on Marécottite and pearly to dull on Meta-autunite.
Often found alongside marécottite
Minerals reported to co-occur with marécottite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₃(UO₂)₈(SO₄)₄(OH)₁₀·28H₂O
- Mohs hardness
- 3
- Density
- 4.4 g/cm³
- Colors
- Streak
- Colorless
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy, rosette, triangular crystals
- Cleavage
- Perfect on {100}; imperfect on {010}
- Fluorescence
- Bright yellow-green under SW UV
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A rare alteration product of selenian digenite in the oxidation zone of a uranium-bearing Cu–Co hydrothermal ore deposit
Where rockhounds find marécottite
Classic worldwide localities
- Musonoi Co–Cu mine, near Kolwezi, Katanga Province, Congo (Shaba Province, Zaire)
Field-hunting tip
Where it forms: A rare alteration product of selenian digenite in the oxidation zone of a uranium-bearing Cu–Co hydrothermal ore deposit. If you start seeing digenite, demesmaekerite, denningite in float, you are in the right ground. Typical habit: platy, rosette, triangular crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify marécottite?+
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Is marécottite safe to handle?+
What rocks look like marécottite?+
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Find marécottite on the map
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