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 5-6, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: grayish black.
- 3Read the lusterHold the specimen under a strong light. Luster: dull to adamantine.
- 4Match the color rangeCompare against the expected color range: black, pale brown, dark brown.
- 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 grayish black, Uraninite leaves brownish black, gray, olive-green, shining; luster reads dull to adamantine on Coffinite and submetallic to greasy, dull on Uraninite.
How to tell apart: Streak differs, so Coffinite leaves grayish black, Thorite leaves light orange to dark brown; luster reads dull to adamantine on Coffinite and vitreous to resinous or greasy 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
- 5-6
- Density
- 5.1 g/cm³
- Streak
- Grayish black
- Luster
- Dull to adamantine
- Transparency
- Opaque, transparent on very thin edges
- Crystal system
- Tetragonal
- Crystal habit
- Massive, fine-grained, microcrystalline aggregates
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector, Ore of uranium
- Host rock
- In Colorado Plateau-type black unoxidized U{V deposits, replacing organic material in sandstone; in other sedimentary and hydrothermal vein uranium deposits
Where rockhounds find coffinite
3 mapped spotsClassic worldwide localities
- La Sal No. 2 mine, Beaver Mesa, Gateway district, Mesa Co., and in the Peanut mine, Montrose Co., Colorado, USA
- Jackpile and Crownpoint mines, Valencia Co., New Mexico, USA
- Mi Vida, Homestake, and other mines, San Juan Co., Utah, USA
- Wölsendorf, Bavaria, Germany
- Niederramstadt, near Schneeberg, Johanngeorgenstadt, and Niederpfannenstiel, Saxony, Germany
- Hüttenberg, Carinthia, Austria
Field-hunting tip
Where it forms: In Colorado Plateau-type black unoxidized U{V deposits, replacing organic material in sandstone; in other sedimentary and hydrothermal vein uranium deposits. If you start seeing uraninite, thorite, pyrite in float, you are in the right ground. Typical habit: massive, fine-grained, microcrystalline aggregates. 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.
