Isoclasite is a rare phosphate mineral typically found as delicate acicular or fibrous crusts in hydrothermal metal deposits. Collectors value it for its tendency to form attractive radiating spray habits that require magnification to fully appreciate.
Is this isoclasite?
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 isoclasite with a known reference. Isoclasite sits at Mohs 3.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Isoclasite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Isoclasite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, colorless.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: fibrous to acicular aggregates, radial clusters.
Often confused with
Isoclasite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside isoclasite
Minerals reported to co-occur with isoclasite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₂PO₄(OH,F)·2H₂O
- Mohs hardness
- 3.5
- Density
- 3.0-3.1 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous to Acicular Aggregates, Radial Clusters
- Cleavage
- Distinct
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Hydrothermal Veins
- Typical price
- $20-150 for micro-specimens
Where rockhounds find isoclasite
Classic worldwide localities
- Jáchymov, Czech Republic
- Wittichen, Germany
- Cornwall, England
Field-hunting tip
Look in hydrothermal veins country — that is the host setting where isoclasite typically forms. If you start seeing arsenopyrite, pharmacosiderite, skutterudite in float, you are in the right ground. Field specimens usually show a fibrous to acicular aggregates, radial clusters habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find isoclasite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
