Coutinhoite is an extremely rare secondary mineral that typically forms as yellowish, earthy crusts or fibrous mats within weathered pegmatites. Due to its thorium and arsenic composition, it is a significant safety concern for collectors, requiring strict containment protocols. It is primarily sought after by advanced systematic mineral collectors specializing in rare species.
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 2-3 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Coutinhoite leaves a yellow streak.
- 3Read the lusterHold the specimen under a strong light. Coutinhoite typically shows a earthy luster.
- 4Match the color rangeCompare against the expected color range: yellow, brownish-yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: fibrous aggregates, crusts.
Often confused with
Coutinhoite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Coutinhoite leaves yellow, Autunite leaves pale yellow; luster reads earthy on Coutinhoite and pearly on Autunite.
How to tell apart: Streak differs — Coutinhoite leaves yellow, Uranophane leaves pale yellow; luster reads earthy 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
- Th₂As₄O₁₂·nH₂O
- Mohs hardness
- 2-3
- Density
- 3.32 g/cm³
- Colors
- Streak
- Yellow
- Luster
- Earthy
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous Aggregates, Crusts
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Granite Pegmatites
- Typical price
- $50-300+ thumbnail
Where rockhounds find coutinhoite
Classic worldwide localities
- Coutinho pegmatite, Minas Gerais, Brazil
Field-hunting tip
Look in granite pegmatites country — that is the host setting where coutinhoite typically forms. If you start seeing monazite, xenotime, zircon in float, you are in the right ground. Field specimens usually show a fibrous aggregates, crusts habit, so 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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