Kroupaite is a rare uranyl sulfate mineral typically found as a secondary mineral in oxidized uranium deposits. It usually occurs as thin, yellow, platy crystals or as earthy crusts associated with other uranium-bearing secondary minerals. Due to its radioactive nature and scarcity, it is sought primarily by advanced mineral collectors.

Hardness

Mohs

Luster

Vitreous

Streak

Light yellow

Transparency

Translucent

mineralrare

Kroupaite

Hardness: ~2
Luster: Vitreous
Streak: Light yellow
Transparency: Translucent

No image

Is this kroupaite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch kroupaite with a known reference. Kroupaite sits at Mohs ~2, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: light yellow.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: orange-yellow.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: platy crystals, crusts, radial aggregates.

Often confused with

Kroupaite vs. its common look-alikes, and how to tell them apart in the field.

mineral

Uranopilite

Mohs 2.5

How to tell apart: Streak differs, so Kroupaite leaves light yellow, Uranopilite leaves pale yellow; luster reads vitreous on Kroupaite and silky on Uranopilite.

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Often found alongside kroupaite

Minerals reported to co-occur with kroupaite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: KPb₀.₅[(UO₂)₈O₄(OH)₁₀]·10H₂O
Mohs hardness: ~2
Density: 5.058 g/cm³
Colors: Orange-yellow
Streak: Light yellow
Luster: Vitreous
Transparency: Translucent
Crystal system: Orthorhombic
Crystal habit: Platy crystals, crusts, radial aggregates
Cleavage: Perfect on {100}
Rarity: Rare
Uses: Collector
Host rock: Of supergene origin by oxidation-hydration alteration of uraninite

Where rockhounds find kroupaite

Classic worldwide localities

Jan Evangelista vein, Daniel level, Svornost mine, Jáchymov, Western Bohemia, Czech Republic

Field-hunting tip

Where it forms: Of supergene origin by oxidation-hydration alteration of uraninite. If you start seeing fourmarierite, na-rich metaschoepite, uranopilite in float, you are in the right ground. Typical habit: platy crystals, crusts, radial aggregates. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify kroupaite?+

Mohs hardness is ~2. Luster: vitreous. The streak is light yellow. Common colors include orange-yellow.

Where is kroupaite found?+

Notable localities include Jan Evangelista vein, Daniel level, Svornost mine, Jáchymov, Western Bohemia, Czech Republic.

Is kroupaite safe to handle?+

Kroupaite contains lead and uranium or thorium and calls for real care. This mineral is radioactive and contains uranium; handle with appropriate safety precautions, including wearing gloves and storing in a shielded container.

What rocks look like kroupaite?+

Kroupaite is most often confused with Uranopilite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with kroupaite?+

Kroupaite commonly co-occurs with Fourmarierite, Na-rich metaschoepite, Uranopilite, Liebigite, Ewingite, Gypsum. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does kroupaite form in?+

Where it forms: Of supergene origin by oxidation-hydration alteration of uraninite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is kroupaite used for?+

Kroupaite is used in collector.

Find kroupaite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.