Compreignacite is a rare secondary uranium mineral often found as small, yellow, tabular crystal aggregates in oxidized uranium deposits. It is specifically known as a product of the alteration of uraninite and requires careful handling due to its radioactivity.
Is this compreignacite?
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 compreignacite with a known reference. Compreignacite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: pale yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: pearly.
- 4Match the color rangeCompare against the expected color range: golden yellow, light yellow.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: tabular crystals, crusts, radial aggregates.
Often confused with
Compreignacite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Compreignacite leaves pale yellow, Becquerelite leaves yellow; luster reads pearly on Compreignacite and adamantine to greasy on Becquerelite.
How to tell apart: Streak differs, so Compreignacite leaves pale yellow, Billietite leaves yellow; luster reads pearly on Compreignacite and adamantine on Billietite.
How to tell apart: Streak differs, so Compreignacite leaves pale yellow, Masuyite leaves orange-yellow; luster reads pearly on Compreignacite and vitreous on Masuyite.
Often found alongside compreignacite
Minerals reported to co-occur with compreignacite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- K₂[(UO₂)₆O₄(OH)₆]·7H₂O
- Mohs hardness
- 2.5
- Density
- 5.03 g/cm³
- Streak
- Pale yellow
- Luster
- Pearly
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular crystals, crusts, radial aggregates
- Cleavage
- Perfect on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- As a very rare oxidation product of “pitchblende” in uranium deposits
Where rockhounds find compreignacite
Classic worldwide localities
- White Canyon, near Moab, Grand Co., Utah, USA
- Marshall Pass district, Saguache Co., Colorado, USA
- Palermo mine, near North Groton, Grafton Co., New Hampshire, USA
- Margnac mine, Compreignac, Haute-Vienne, France
- Wheal Owles and Wheal Edward, St. Just, Cornwall, England
- Jáchymov (Joachimsthal), Czech Republic
Field-hunting tip
Where it forms: As a very rare oxidation product of “pitchblende” in uranium deposits. If you start seeing uraninite, cuprosklodowskite, uranophane in float, you are in the right ground. Typical habit: tabular crystals, crusts, radial aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify compreignacite?+
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What rocks look like compreignacite?+
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Find compreignacite on the map
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