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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
3
Mohs
Luster
Pearly
Streak
Yellow
Transparency
Translucent

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.

Uranopilite mineral
mineral
Uranopilite
Mohs 2.5

How to tell apart: Streak differs — Kroupaite leaves yellow, Uranopilite leaves pale yellow.

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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
K₂[UO₂(SO₄)₂]·nH₂O
Mohs hardness
3
Density
4.92 g/cm³
Colors
YellowOrange-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 — that is 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

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