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, so Coutinhoite leaves yellow, Autunite leaves pale yellow; luster reads earthy on Coutinhoite and pearly on Autunite.
How to tell apart: Streak differs, so 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, 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
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
