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

Pearly

Streak

Yellow

Transparency

Translucent

mineralrare

Kroupaite

Hardness: 3
Luster: Pearly
Streak: Yellow
Transparency: Translucent

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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 3, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Kroupaite leaves a yellow streak.
3
Read the luster
Hold the specimen under a strong light. Kroupaite typically shows a pearly luster.
4
Match the color range
Compare against the expected color range: yellow, 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 yellow, Uranopilite leaves pale yellow.

Compare →

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: K₂[UO₂(SO₄)₂]·nH₂O
Mohs hardness: 3
Density: 4.92 g/cm³
Colors: Yellow Orange-yellow
Streak: Yellow
Luster: Pearly
Transparency: Translucent
Crystal system: Orthorhombic
Crystal habit: Platy Crystals, Crusts, Radial Aggregates
Cleavage: Perfect
Rarity: Rare
Uses: Collector
Host rock: Hydrothermal Uranium Veins
Typical price: $50-300 per specimen

Where rockhounds find kroupaite

Classic worldwide localities

Jáchymov, Czech Republic
Loděnice, Czech Republic

Field-hunting tip

Look in hydrothermal uranium veins country, the host setting where kroupaite typically forms. If you start seeing uraninite, gummite, uranopilite in float, you are in the right ground. Field specimens usually show a platy crystals, crusts, radial aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify kroupaite?+

Mohs hardness is 3. It typically shows a pearly luster. The streak is yellow. Common colors include yellow, orange-yellow.

Where is kroupaite found?+

Notable localities include Jáchymov, Czech Republic; Loděnice, Czech Republic.

How much is kroupaite worth?+

Typical asking prices fall in the range of $50-300 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

Is kroupaite safe to handle?+

This mineral is radioactive. It contains toxic constituents. This mineral is radioactive and contains uranium; handle with appropriate safety precautions, including wearing gloves and storing in a shielded container. Handle with care, avoid grinding or breathing dust, and store separately.

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 Uraninite, Gummite, Uranopilite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does kroupaite form in?+

Kroupaite typically forms in hydrothermal uranium veins. 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.