Grimselite is a rare uranyl carbonate mineral typically found as small, bright yellow to yellow-green tabular crystals. It is most famous from its type locality at the Grimsel Pass in Switzerland, where it occurs as a secondary mineral on granite surfaces. Collectors value it for its vibrant appearance and strong fluorescence under ultraviolet light.

Hardness

2-2.5

Mohs

Luster

Vitreous

Streak

Pale yellow

Transparency

Translucent to transparent

mineralrare

Grimselite

Hardness: 2-2.5
Luster: Vitreous
Streak: Pale yellow
Transparency: Translucent to transparent

Is this grimselite?

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 grimselite with a known reference. Grimselite sits at Mohs 2-2.5, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: pale 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: yellow.
5
Look at form & habit
Crystal system: hexagonal. Typical habit: tabular crystals, crusts, or coatings.

Often confused with

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

mineral

Liebigite

Mohs 2-3

How to tell apart: Streak differs, so Grimselite leaves pale yellow, Liebigite leaves yellow white.

How to tell apart: Luster reads vitreous on Grimselite and subvitreous, pearly on {001} on Schröckingerite.

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Often found alongside grimselite

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

All properties

Chemical formula: K₃Na(UO₂)(CO₃)₃·H₂O
Mohs hardness: 2-2.5
Density: 3.3 g/cm³
Colors: Yellow
Streak: Pale yellow
Luster: Vitreous
Transparency: Translucent to transparent
Crystal system: Hexagonal
Crystal habit: Tabular crystals, crusts, or coatings
Cleavage: None
Fluorescence: Bright green under UV
Rarity: Rare
Uses: Collector
Host rock: A rare secondary mineral in veins in mineralized granodiorite

Where rockhounds find grimselite

Classic worldwide localities

Gerstenegg (Kabelstollen Gerstenegg-Grimsel I), Grimsel, Bern, Switzerland (type locality)

Field-hunting tip

Where it forms: A rare secondary mineral in veins in mineralized granodiorite. If you start seeing schrockingerite, baylissite, monohydrocalcite in float, you are in the right ground. Typical habit: tabular crystals, crusts, or coatings. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify grimselite?+

Mohs hardness is 2-2.5. Luster: vitreous. The streak is pale yellow. Common colors include yellow.

Where is grimselite found?+

Notable localities include Gerstenegg (Kabelstollen Gerstenegg-Grimsel I), Grimsel, Bern, Switzerland (type locality).

Is grimselite safe to handle?+

Grimselite contains uranium or thorium and calls for real care. Contains uranium; handle with gloves, avoid inhalation of dust, and store in a lead-lined or shielded container away from other mineral specimens.

What rocks look like grimselite?+

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

What minerals are found with grimselite?+

Grimselite commonly co-occurs with Schrockingerite, Baylissite, Monohydrocalcite, Calcite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does grimselite form in?+

Where it forms: A rare secondary mineral in veins in mineralized granodiorite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is grimselite used for?+

Grimselite is used in collector.

Find grimselite on the map

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