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 — 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 — 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 — that is 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.
