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.
Is this preiswerkite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch preiswerkite with a known reference. Preiswerkite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to pearly.
- 4Match the color rangeCompare against the expected color range: greenish white, light green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: platy crystals.
Often confused with
Preiswerkite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads vitreous to pearly on Preiswerkite and pearly on Paragonite.
How to tell apart: Margarite is the harder of the two (Mohs 3.5-4.5 vs. 2.5); luster reads vitreous to pearly on Preiswerkite and pearly on cleavage, vitreous on lateral faces on Margarite.
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
- Density
- 2.96 g/cm³
- Streak
- White
- Luster
- Vitreous to pearly
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Platy crystals
- Cleavage
- Perfect on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Formed in a rodingite dike during upper-greenschist to lower-amphibolite facies metamorphism (Geisspfad complex, Switzerland)
Where rockhounds find preiswerkite
Classic worldwide localities
- Geisspfad ultramafic complex, Binntal, Valais, Switzerland
- Liset, Selje district, southwestern Norway
Field-hunting tip
Where it forms: Formed in a rodingite dike during upper-greenschist to lower-amphibolite facies metamorphism (Geisspfad complex, Switzerland). If you start seeing aluminian pargasite, zoisite in float, you are in the right ground. Typical habit: platy crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify preiswerkite?+
Where is preiswerkite found?+
What rocks look like preiswerkite?+
What minerals are found with preiswerkite?+
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What is preiswerkite used for?+
Find preiswerkite on the map
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