Fontanite is a rare uranyl carbonate mineral typically found as secondary crusts or delicate platy crystals in uranium deposits. It is recognized by its soft, pearly appearance and association with other secondary uranium minerals like autunite.
Is this fontanite?
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 fontanite with a known reference. Fontanite 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: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: honey yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: aggregates, radial.
Often confused with
Fontanite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Fontanite leaves white, Schröckingerite leaves pale yellow; luster reads vitreous on Fontanite and subvitreous, pearly on {001} on Schröckingerite.
How to tell apart: Streak differs, so Fontanite leaves white, Liebigite leaves yellow white.
Often found alongside fontanite
Minerals reported to co-occur with fontanite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca(UO₂)₃(CO₃)₂O₂·6H₂O
- Mohs hardness
- 3
- Density
- 4.10 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Aggregates, radial
- Cleavage
- Good on {100}, {010}, and {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A rare secondary mineral in the oxidized portions of a uranium deposit
Where rockhounds find fontanite
Classic worldwide localities
- Rabéjac uranium deposit, seven km south-southeast of Lodève, Hérault, France
Field-hunting tip
Where it forms: A rare secondary mineral in the oxidized portions of a uranium deposit. If you start seeing billietite, uranophane in float, you are in the right ground. Typical habit: aggregates, radial. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify fontanite?+
Where is fontanite found?+
Is fontanite safe to handle?+
What rocks look like fontanite?+
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Find fontanite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
