Weeksite is a distinctive radioactive uranium mineral typically found as radiating clusters of yellow, fan-shaped crystals or crusts. It is highly prized by collectors of radioactive species for its vibrant yellow-green fluorescence under ultraviolet light.
Is this weeksite?
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 weeksite with a known reference. Weeksite sits at Mohs <2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: waxy to silky.
- 4Match the color rangeCompare against the expected color range: yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular, bladed, platy.
Often confused with
Weeksite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Boltwoodite is the harder of the two (Mohs 3.5-4 vs. <2); streak differs, so Weeksite leaves yellow, Boltwoodite leaves white; luster reads waxy to silky on Weeksite and pearly; radial aggregates vitreous to silky; dull to earthy in microcrystalline pseudomorphs on Boltwoodite.
How to tell apart: Soddyite is the harder of the two (Mohs 3.5 vs. <2); streak differs, so Weeksite leaves yellow, Soddyite leaves pale yellow; luster reads waxy to silky on Weeksite and vitreous to adamantine, dull when earthy on Soddyite.
Often found alongside weeksite
Minerals reported to co-occur with weeksite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- K₂(UO₂)₂(Si₅O₁₃)·4H₂O
- Mohs hardness
- <2
- Density
- 4.1 g/cm³
- Colors
- Streak
- Yellow
- Luster
- Waxy to silky
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular, bladed, platy
- Cleavage
- Two good prismatic cleavages noted
- Fluorescence
- Bright yellow-green under UV
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- In “opal" veinlets in rhyolite, agglomerates, sandstones and limestones
Where rockhounds find weeksite
Classic worldwide localities
- Autunite No. 8 and Good Will claims, Thomas Range, Juab Co., Utah, USA
- Coso Mountains, Inyo Co., and in the Red Rock district, Lassen Co., California, USA
- Jackpile mine, Laguna, Valencia Co., New Mexico, USA
- Silver Cliff mine, near Lusk, Niobrara Co., Wyoming, USA
- Margaritas and other mines, Sierra Peña Blanca, Chihuahua, Mexico
- Les Bois-Noirs, Loire, France
Field-hunting tip
Where it forms: In “opal" veinlets in rhyolite, agglomerates, sandstones and limestones. If you start seeing opal, chalcedony, calcite in float, you are in the right ground. Typical habit: acicular, bladed, platy. Train your eye for that shape before scanning the outcrop.
Common questions
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Find weeksite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
