Zwieselite is a relatively rare phosphate mineral typically found as massive or granular aggregates within phosphate-rich pegmatites. It is visually similar to triplite but distinguished by its distinct chemical composition; it is primarily found in granite pegmatites associated with other phosphate minerals.
Is this zwieselite?
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 zwieselite with a known reference. Zwieselite sits at Mohs 5-5.5, 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 resinous.
- 4Match the color rangeCompare against the expected color range: dark brown, black, black brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: pseudo cubic.
Often confused with
Zwieselite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside zwieselite
Minerals reported to co-occur with zwieselite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe²⁺₂PO₄F
- Mohs hardness
- 5-5.5
- Density
- 3.89-3.97 g/cm³
- Streak
- White
- Luster
- Vitreous to resinous
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
- Crystal habit
- Pseudo cubic
- Cleavage
- On {001}, good; on {010}, fair; on {100}, poor (by analogy to triplite)
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- A primary phosphate in complex zoned granite pegmatites
Where rockhounds find zwieselite
Classic worldwide localities
- Storm Mountain pegmatite, Larimer Co., Colorado, USA
- Bavaria, from Hühnerkobel and in the Birkhöhe pegmatite, near Zwiesel, Germany
- Hörlberg, Germany
- Döfering, Germany
- Hagendorf, Germany
- Horní Slavkov (Schlaggenwald), Czech Republic
Field-hunting tip
Where it forms: A primary phosphate in complex zoned granite pegmatites. If you start seeing triphylite, huréaulite, rockbridgeite in float, you are in the right ground. Typical habit: pseudo cubic. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify zwieselite?+
Where is zwieselite found?+
What rocks look like zwieselite?+
What minerals are found with zwieselite?+
What kind of rock does zwieselite form in?+
What is zwieselite used for?+
Find zwieselite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
