Coutinhoite is a rare thorium barium uranium silicate that forms yellow irregular aggregates of curved flaky crystals. It is a secondary hydrothermal mineral in a uraninite-bearing granitic pegmatite. It was first described from Urucum mine, Galiléia Co., Minas Gerais, Brazil.
Is this coutinhoite?
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 coutinhoite with a known reference. Coutinhoite 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: waxy to silky.
- 4Match the color rangeCompare against the expected color range: yellow.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: flakes, scaly.
Often confused with
Coutinhoite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Coutinhoite leaves yellow, Autunite leaves pale yellow; luster reads waxy to silky on Coutinhoite and vitreous, pearly on {001} on Autunite.
How to tell apart: Streak differs, so Coutinhoite leaves yellow, Uranophane leaves yellowish white; luster reads waxy to silky on Coutinhoite and vitreous on Uranophane.
Often found alongside coutinhoite
Minerals reported to co-occur with coutinhoite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- ThₓBa₁₋₂ₓ(UO₂)₂Si₅O₁₃·3H₂O
- Mohs hardness
- <2
- Density
- 3.839 g/cm³
- Colors
- Streak
- Yellow
- Luster
- Waxy to silky
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Flakes, scaly
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A secondary hydrothermal mineral in a uraninite-bearing granitic pegmatite
Where rockhounds find coutinhoite
Classic worldwide localities
- Urucum mine, Galiléia Co., Minas Gerais, Brazil
Field-hunting tip
Where it forms: A secondary hydrothermal mineral in a uraninite-bearing granitic pegmatite. If you start seeing weeksite, phosphuranylite, metauranocircite in float, you are in the right ground. Typical habit: flakes, scaly. Train your eye for that shape before scanning the outcrop.
Common questions
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Find coutinhoite on the map
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