Preiswerkite is a rare trioctahedral mica found primarily in meta-ultramafic rocks. It is most easily identified by its platy habit and occurrence in serpentinite environments, often appearing as pale, micaceous flakes similar to other micas.

Hardness

2.5-3

Mohs

Luster

Pearly

Streak

White

Transparency

Translucent

mineralrareMica Group family

Preiswerkite

Hardness: 2.5-3
Luster: Pearly
Streak: White
Transparency: Translucent

Is this preiswerkite?

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 preiswerkite with a known reference. Preiswerkite sits at Mohs 2.5-3, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Preiswerkite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Preiswerkite typically shows a pearly luster.
4
Match the color range
Compare against the expected color range: white, colorless, pale yellow.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: platy crystals.

Often confused with

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

Often found alongside preiswerkite

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

All properties

Chemical formula: NaMg₂Al₃Si₂O₁₀(OH)₂
Mohs hardness: 2.5-3
Density: 2.85 g/cm³
Colors: White Colorless Pale Yellow
Streak: White
Luster: Pearly
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Platy Crystals
Cleavage: Perfect Basal
Rarity: Rare
Uses: Collector
Host rock: Metamorphic Serpentinite
Typical price: $20-100 per specimen

Where rockhounds find preiswerkite

Classic worldwide localities

Grisons, Switzerland
Val d'Ala, Italy

Field-hunting tip

Look in metamorphic serpentinite country, the host setting where preiswerkite typically forms. If you start seeing chlorite, calcite, talc in float, you are in the right ground. Field specimens usually show a platy crystals habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify preiswerkite?+

Mohs hardness is 2.5-3. It typically shows a pearly luster. The streak is white. Common colors include white, colorless, pale yellow.

Where is preiswerkite found?+

Notable localities include Grisons, Switzerland; Val d'Ala, Italy.

How much is preiswerkite worth?+

Typical asking prices fall in the range of $20-100 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like preiswerkite?+

Preiswerkite is most often confused with Muscovite, Paragonite, Margarite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with preiswerkite?+

Preiswerkite commonly co-occurs with Chlorite, Calcite, Talc, Chromite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does preiswerkite form in?+

Preiswerkite typically forms in metamorphic serpentinite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is preiswerkite used for?+

Preiswerkite is used in collector.

Find preiswerkite on the map

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