Kruijenite is an extremely rare sulfate mineral originally discovered in the Netherlands. It typically forms small, clear, tabular crystals within altered limestone xenoliths and is primarily sought after by advanced systematic mineral collectors.

Hardness

Mohs

Luster

Vitreous

Transparency

Transparent to translucent

mineralrare

Kruijenite

Hardness: 3
Luster: Vitreous
Transparency: Transparent to translucent

Is this kruijenite?

4-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 kruijenite with a known reference. Kruijenite sits at Mohs 3, softer than the next harder reference, harder than the previous one.
2
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
3
Match the color range
Compare against the expected color range: colorless, pale greenish yellow.
4
Look at form & habit
Crystal system: tetragonal. Typical habit: tabular crystals.

Often confused with

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

mineral

Gypsum

Mohs 1.5-2

How to tell apart: Kruijenite is noticeably harder (Mohs 3 vs. 1.5-2); luster reads vitreous on Kruijenite and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.

Often found alongside kruijenite

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

All properties

Chemical formula: Ca₄Al₄(SO₄)F₂(OH)₁₆·2H₂O
Mohs hardness: 3
Density: 2.573 g/cm³
Colors: Colorless Pale greenish yellow
Luster: Vitreous
Transparency: Transparent to translucent
Crystal system: Tetragonal
Crystal habit: Tabular crystals
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: In fragments of altered calcic xenolith from tephra, the product of pyrometamorphic and hydrothermal alteration by magmatic volatiles

Where rockhounds find kruijenite

Classic worldwide localities

Feuerberg quarry, 7 km north of Daun, Hinterweiler municipality, Eifel paleovolcanic region, Rhineland-Palatinate, Germany

Field-hunting tip

Where it forms: In fragments of altered calcic xenolith from tephra, the product of pyrometamorphic and hydrothermal alteration by magmatic volatiles. If you start seeing fluorite, calcite, aragonite in float, you are in the right ground. Typical habit: tabular crystals. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify kruijenite?+

Mohs hardness is 3. Luster: vitreous. Common colors include colorless, pale greenish yellow.

Where is kruijenite found?+

Notable localities include Feuerberg quarry, 7 km north of Daun, Hinterweiler municipality, Eifel paleovolcanic region, Rhineland-Palatinate, Germany.

What rocks look like kruijenite?+

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

What minerals are found with kruijenite?+

Kruijenite commonly co-occurs with Fluorite, Calcite, Aragonite, Cuspidine, Magnesioferrite, Hematite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does kruijenite form in?+

Where it forms: In fragments of altered calcic xenolith from tephra, the product of pyrometamorphic and hydrothermal alteration by magmatic volatiles. Working float back to the host body is the standard way to chase a fresh occurrence.

What is kruijenite used for?+

Kruijenite is used in collector.

Find kruijenite on the map

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