Hallimondite is a very rare lead uranyl arsenate mineral typically found as small, bright yellow tabular crystals. It is a secondary mineral primarily occurring in the oxidation zones of uranium-bearing hydrothermal deposits, often associated with other rare secondary uranium minerals.
Is this hallimondite?
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 hallimondite with a known reference. Hallimondite sits at Mohs 2.5-3, 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: subadamantine.
- 4Match the color rangeCompare against the expected color range: yellow.
- 5Look at form & habitCrystal system: triclinic. Typical habit: platy crystals, tabular.
Often confused with
Hallimondite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads subadamantine on Hallimondite and vitreous, pearly on {001} on Autunite.
How to tell apart: Luster reads subadamantine on Hallimondite and vitreous, subadamantine, waxy, pearly on cleavages; becoming dull on dehydration to metatorbernite on Torbernite.
Often found alongside hallimondite
Minerals reported to co-occur with hallimondite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Pb₂(UO₂)(AsO₄)₂·nH₂O
- Mohs hardness
- 2.5-3
- Density
- 6.39 g/cm³
- Colors
- Streak
- Pale yellow
- Luster
- Subadamantine
- Transparency
- Transparent to translucent
- Crystal system
- Triclinic
- Crystal habit
- Platy crystals, tabular
- Cleavage
- Indistinct
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A secondary mineral found on a museum specimen from an oxidizing As-Pb-bearing deposit, formed by alteration of galena
Where rockhounds find hallimondite
Classic worldwide localities
- Michael mine, Weiler, near Lahr, Black Forest, and on the Biihlskopf, near Ellweiler, Rhineland-Palatinate, Germany
Field-hunting tip
Where it forms: A secondary mineral found on a museum specimen from an oxidizing As-Pb-bearing deposit, formed by alteration of galena. If you start seeing hiigelite, mimetite, barite in float, you are in the right ground. Typical habit: platy crystals, tabular. Train your eye for that shape before scanning the outcrop.
Common questions
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Find hallimondite on the map
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