Coffinite is a primary uranium silicate mineral that rarely forms distinct crystals, usually appearing as black, massive, or earthy masses. It is notoriously difficult to identify without X-ray diffraction due to its resemblance to pitchblende and amorphous nature, often requiring professional testing for positive identification.
Is this coffinite?
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 coffinite with a known reference. Coffinite sits at Mohs 5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Coffinite leaves a brownish-black streak.
- 3Read the lusterHold the specimen under a strong light. Coffinite typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: black, dark gray, brownish-black.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: massive, fine-grained, microcrystalline aggregates.
Often confused with
Coffinite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Coffinite leaves brownish-black, Uraninite leaves brownish-black to greenish-black; luster reads dull on Coffinite and submetallic to dull on Uraninite.
How to tell apart: Luster reads dull on Coffinite and submetallic on Uranium Ore.
How to tell apart: Streak differs, so Coffinite leaves brownish-black, Thorite leaves dark brown; luster reads dull on Coffinite and resinous on Thorite.
Often found alongside coffinite
Minerals reported to co-occur with coffinite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- U(SiO₄)₁₋ₓ(OH)₄ₓ
- Mohs hardness
- 5
- Density
- 5.1-5.4 g/cm³
- Streak
- Brownish-black
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Tetragonal
- Crystal habit
- Massive, Fine-grained, Microcrystalline Aggregates
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector, Ore of Uranium
- Host rock
- Sedimentary Sandstone Uranium Deposits, Hydrothermal Veins
- Typical price
- $20-150 thumbnail, $200+ cabinet specimen
Where rockhounds find coffinite
3 mapped spotsClassic worldwide localities
- Colorado Plateau, USA
- Schlema-Hartenstein, Germany
- Witwatersrand, South Africa
- Rum Jungle, Australia
Field-hunting tip
Look in sedimentary sandstone uranium deposits, hydrothermal veins country, the host setting where coffinite typically forms. If you start seeing uraninite, pyrite, marcasite in float, you are in the right ground. Field specimens usually show a massive, fine-grained, microcrystalline aggregates habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah, Wyoming. Start trip planning there.
Common questions
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Find coffinite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
