Rokühnite is a very rare iron chloride hydrate that typically forms as a secondary mineral in oxidized iron-ore deposits. Because it is highly deliquescent and sensitive to humidity, it must be stored in a sealed, dry environment to prevent it from absorbing moisture and dissolving.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Semitransparent

mineralrare

Rokühnite

Hardness: 2
Luster: Vitreous
Streak: White
Transparency: Semitransparent

Is this rokühnite?

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 rokühnite with a known reference. Rokühnite sits at Mohs 2, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: colorless.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: tabular crystals, crusts.

Often confused with

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

How to tell apart: Streak differs, so Rokühnite leaves white, Erythrite leaves pale red to pink; luster reads vitreous on Rokühnite and subadamantine, pearly on cleavages on Erythrite.

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Often found alongside rokühnite

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

All properties

Chemical formula: FeCl₂·2H₂O
Mohs hardness: 2
Density: 2.35 g/cm³
Colors: Colorless
Streak: White
Luster: Vitreous
Transparency: Semitransparent
Crystal system: Monoclinic
Crystal habit: Tabular crystals, crusts
Cleavage: Very good on {110}; good on {010}
Rarity: Rare
Uses: Collector
Host rock: A secondary mineral in fissures in earlier potassium-rich salt beds

Where rockhounds find rokühnite

Classic worldwide localities

Salzdetfurth potash mine, Hamelyn, and the Siegfried-Giesen and Hope potash mines, Sarstedt, Zechstein basin, south of Hannover, Lower Saxony, Germany

Field-hunting tip

Where it forms: A secondary mineral in fissures in earlier potassium-rich salt beds. If you start seeing carnallite, rinneite, halite in float, you are in the right ground. Typical habit: tabular crystals, crusts. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify rokühnite?+

Mohs hardness is 2. Luster: vitreous. The streak is white. Common colors include colorless.

Where is rokühnite found?+

Notable localities include Salzdetfurth potash mine, Hamelyn, and the Siegfried-Giesen and Hope potash mines, Sarstedt, Zechstein basin, south of Hannover, Lower Saxony, Germany.

What rocks look like rokühnite?+

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

What minerals are found with rokühnite?+

Rokühnite commonly co-occurs with Carnallite, Rinneite, Halite, Tachyhydrite, Sylvite, Langbeinite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does rokühnite form in?+

Where it forms: A secondary mineral in fissures in earlier potassium-rich salt beds. Working float back to the host body is the standard way to chase a fresh occurrence.

What is rokühnite used for?+

Rokühnite is used in collector.

Find rokühnite on the map

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