Kottenheimite is a very rare hydrated calcium sulfate hydroxide mineral discovered in the volcanic fields of the Eifel region in Germany. It typically forms as delicate, white platy crystals or efflorescent crusts within cavities of volcanic rock. Due to its solubility and rarity, collectors should handle specimens with care and store them in stable, humidity-controlled conditions.

Hardness

2-2.5

Mohs

Luster

Vitreous to silky (aggregates)

Transparency

Translucent

mineralrare

Kottenheimite

Hardness: 2-2.5
Luster: Vitreous to silky (aggregates)
Transparency: Translucent

Is this kottenheimite?

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 kottenheimite with a known reference. Kottenheimite sits at Mohs 2-2.5, softer than the next harder reference, harder than the previous one.
2
Read the luster
Hold the specimen under a strong light. Luster: vitreous to silky (aggregates).
3
Match the color range
Compare against the expected color range: snow-white.
4
Look at form & habit
Crystal system: hexagonal. Typical habit: platy crystals, pseudohexagonal, efflorescent crusts.

Often confused with

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

mineral

Gypsum

Mohs 1.5-2

How to tell apart: Luster reads vitreous to silky (aggregates) on Kottenheimite and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.

Often found alongside kottenheimite

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

All properties

Chemical formula: Ca₃Si(SO₄)₂(OH)₆·12H₂O
Mohs hardness: 2-2.5
Density: 1.92 g/cm³
Colors: Snow-white
Luster: Vitreous to silky (aggregates)
Transparency: Translucent
Crystal system: Hexagonal
Crystal habit: Platy crystals, pseudohexagonal, efflorescent crusts
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: In miarolitic cavities in alkali basalt

Where rockhounds find kottenheimite

Classic worldwide localities

Bellerberg, near Kottenheim, Eastern Eifel area, Rhineland-Palatinate (Rheinland-Pfalz), Germany

Field-hunting tip

Where it forms: In miarolitic cavities in alkali basalt. If you start seeing wollastonite, clinochlore, ellestadite in float, you are in the right ground. Typical habit: platy crystals, pseudohexagonal, efflorescent crusts. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify kottenheimite?+

Mohs hardness is 2-2.5. Luster: vitreous to silky (aggregates). Common colors include snow-white.

Where is kottenheimite found?+

Notable localities include Bellerberg, near Kottenheim, Eastern Eifel area, Rhineland-Palatinate (Rheinland-Pfalz), Germany.

What rocks look like kottenheimite?+

Kottenheimite 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 kottenheimite?+

Kottenheimite commonly co-occurs with Wollastonite, Clinochlore, Ellestadite, Melilite, Cuspidine, Earlier-formed sanidine. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does kottenheimite form in?+

Where it forms: In miarolitic cavities in alkali basalt. Working float back to the host body is the standard way to chase a fresh occurrence.

What is kottenheimite used for?+

Kottenheimite is used in collector.

Find kottenheimite on the map

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