Calcioursilite is a rare secondary uranium mineral that forms in the oxidized zones of uranium deposits. It is best identified by its bright yellow color and distinct fluorescence, but it requires careful handling due to its radioactive nature.

Hardness

2-3

Mohs

Luster

Pearly

Streak

Pale Yellow

Transparency

Translucent

mineralrareUranyl silicate group family

Calcioursilite

Hardness: 2-3
Luster: Pearly
Streak: Pale Yellow
Transparency: Translucent

No image

Is this calcioursilite?

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 calcioursilite with a known reference. Calcioursilite sits at Mohs 2-3 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Calcioursilite leaves a pale yellow streak.
3
Read the luster
Hold the specimen under a strong light. Calcioursilite typically shows a pearly luster.
4
Match the color range
Compare against the expected color range: yellow, golden yellow.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: platy crystals, fibrous, earthy masses.

Often confused with

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

mineral

Uranophane

Mohs 2.5

How to tell apart: Luster reads pearly on Calcioursilite and vitreous on Uranophane.

How to tell apart: Streak differs — Calcioursilite leaves pale yellow, Boltwoodite leaves yellow.

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Often found alongside calcioursilite

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

All properties

Chemical formula: Ca(UO₂)₅(Si₂O₅)₃(OH)₂·~16H₂O
Mohs hardness: 2-3
Density: 3.3-3.6 g/cm³
Colors: Yellow Golden Yellow
Streak: Pale Yellow
Luster: Pearly
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Platy Crystals, Fibrous, Earthy Masses
Cleavage: Perfect Basal
Fluorescence: Bright Yellow-green Under UV
Rarity: Rare
Uses: Collector
Host rock: Oxidized Zones of Uranium-bearing Hydrothermal Deposits
Typical price: $50-300 per specimen depending on size and intensity of radiation

Where rockhounds find calcioursilite

Classic worldwide localities

Krivoy Rog, Ukraine
Wölsendorf, Germany
Lodève, France

Field-hunting tip

Look in oxidized zones of uranium-bearing hydrothermal deposits country — that is the host setting where calcioursilite typically forms. If you start seeing uraninite, autunite, quartz in float, you are in the right ground. Field specimens usually show a platy crystals, fibrous, earthy masses habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify calcioursilite?+

Mohs hardness is 2-3. It typically shows a pearly luster. The streak is pale yellow. Common colors include yellow, golden yellow.

Where is calcioursilite found?+

Notable localities include Krivoy Rog, Ukraine; Wölsendorf, Germany; Lodève, France.

How much is calcioursilite worth?+

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

Is calcioursilite safe to handle?+

This mineral is radioactive. It contains toxic constituents. This mineral is radioactive and contains uranium; handle with care, wash hands thoroughly after contact, and store in a lead-lined container away from living spaces. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like calcioursilite?+

Calcioursilite is most often confused with Uranophane, Boltwoodite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with calcioursilite?+

Calcioursilite commonly co-occurs with Uraninite, Autunite, Quartz, Calcite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does calcioursilite form in?+

Calcioursilite typically forms in oxidized zones of uranium-bearing hydrothermal deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is calcioursilite used for?+

Calcioursilite is used in collector.

Find calcioursilite on the map

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