Fransoletite is a rare phosphate mineral typically found as white, bladed crystals or radial aggregates in phosphate-rich pegmatites. It is highly sought after by advanced micromount collectors due to its rarity and specific formation conditions in altered beryllonite zones.
Is this fransoletite?
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 fransoletite with a known reference. Fransoletite sits at Mohs ~3, 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: subvitreous.
- 4Match the color rangeCompare against the expected color range: white, colorless.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: bladed crystals, radial aggregates, crusts.
Often confused with
Fransoletite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Beryllonite is the harder of the two (Mohs 5.5-6 vs. ~3); luster reads subvitreous on Fransoletite and vitreous to adamantine, may be pearly on {010} on Beryllonite.
How to tell apart: Herderite is the harder of the two (Mohs 5-5.5 vs. ~3); luster reads subvitreous on Fransoletite and vitreous on Herderite.
Often found alongside fransoletite
Minerals reported to co-occur with fransoletite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₃Be₂(PO₄)₂(PO₃OH)₂·4H₂O
- Mohs hardness
- ~3
- Density
- 2.56 g/cm³
- Streak
- White
- Luster
- Subvitreous
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Bladed crystals, radial aggregates, crusts
- Cleavage
- {010}, imperfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Formed by late-stage hydrothermal alteration of beryl and other phosphates in a zoned granite pegmatite
Where rockhounds find fransoletite
Classic worldwide localities
- Tip Top mine, 8.5 km southwest of Custer, Custer Co., South Dakota, USA
Field-hunting tip
Where it forms: Formed by late-stage hydrothermal alteration of beryl and other phosphates in a zoned granite pegmatite. If you start seeing beryl, tiptopite, whitlockite in float, you are in the right ground. Typical habit: bladed crystals, radial aggregates, crusts. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify fransoletite?+
Where is fransoletite found?+
Is fransoletite safe to handle?+
What rocks look like fransoletite?+
What minerals are found with fransoletite?+
What kind of rock does fransoletite form in?+
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Find fransoletite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
