Horákite is a rare bismuth uranium arsenate, typically greenish yellow to pale yellow. It is an oxidation zone, secondary mineral in vugs in quartz veinlets with relics of tennantite and uraninite. It is associated with phosphuranylite and metatorbernite-metazeunerite. It was first described from Jáchymov (St. Joachimsthal), Czech Republic (most likely from the Geister vein at the Rovnost mine).
Is this horákite?
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 horákite with a known reference. Horákite 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: light yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: greenish yellow, pale yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic to tabular crystals.
Often confused with
Horákite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Triplite is the harder of the two (Mohs 5-5.5 vs. ~2); streak differs, so Horákite leaves light yellow, Triplite leaves white to brown.
How to tell apart: Amblygonite is the harder of the two (Mohs 5.5-6 vs. ~2); streak differs, so Horákite leaves light yellow, Amblygonite leaves white.
Often found alongside horákite
Minerals reported to co-occur with horákite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Bi₇O₇OH)[(UO₂)₄(PO₄)₂(AsO₄)₂(OH)₂]·3.5H₂O
- Mohs hardness
- ~2
- Density
- 6.538 g/cm³
- Streak
- Light yellow
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic to tabular crystals
- Cleavage
- Perfect on {100}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- An oxidation zone, secondary mineral in vugs in quartz veinlets with relics of tennantite and uraninite
Where rockhounds find horákite
Classic worldwide localities
- Jáchymov (St. Joachimsthal), Czech Republic (most likely from the Geister vein at the Rovnost mine)
Field-hunting tip
Where it forms: An oxidation zone, secondary mineral in vugs in quartz veinlets with relics of tennantite and uraninite. If you start seeing phosphuranylite, metatorbernite-metazeunerite in float, you are in the right ground. Typical habit: prismatic to tabular crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify horákite?+
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What rocks look like horákite?+
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Find horákite on the map
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