Frondelite is a rare phosphate mineral typically found as radial sprays of needle-like crystals or botryoidal masses in phosphate-rich pegmatites. Collectors prize its rich yellow to reddish-brown coloration and distinct fibrous habit. It is often identified as an alteration product of primary triphylite in complex pegmatite zones.
Is this frondelite?
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 frondelite with a known reference. Frondelite sits at Mohs 4.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: brownish gray.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to dull.
- 4Match the color rangeCompare against the expected color range: brown, brown green, brownish black, green black, dark green.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: fibrous, radial, botryoidal, or acicular crystals.
Often confused with
Frondelite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Frondelite leaves brownish gray, Rockbridgeite leaves greenish gray.
How to tell apart: Streak differs, so Frondelite leaves brownish gray, Dufrénite leaves green.
How to tell apart: Frondelite is noticeably harder (Mohs 4.5 vs. 3.5); streak differs, so Frondelite leaves brownish gray, Strengite leaves white.
Often found alongside frondelite
Minerals reported to co-occur with frondelite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mn²⁺Fe³⁺₄(PO₄)₃(OH)₅
- Mohs hardness
- 4.5
- Density
- 3.453 g/cm³
- Colors
- Streak
- Brownish gray
- Luster
- Vitreous to dull
- Transparency
- Subtranslucent
- Crystal system
- Orthorhombic
- Crystal habit
- Fibrous, radial, botryoidal, or acicular crystals
- Cleavage
- {100}, excellent; {010}, good; {001}, fair
- Rarity
- Rare
- Uses
- Collector
- Host rock
- An alteration product of primary phosphates in complex granite pegmatites
Where rockhounds find frondelite
Classic worldwide localities
- Fletcher mine, near North Groton, Grafton Co., New Hampshire, and in the Tip Top mine, 8.5 km southwest of Custer, Custer Co., USA
- Sapucaia pegmatite mine, about 50 km east-southeast of Governador Valadares, Minas Gerais, Brazil
- Hagendorf, Bavaria, Germany
- Buranga pegmatite, near Gatumba, Rwanda
- Kobokobo pegmatite, Lusungu River district, Kivu Province, Congo (Zaire)
Field-hunting tip
Where it forms: An alteration product of primary phosphates in complex granite pegmatites. If you start seeing triphylite, vivianite, heterosite–purpurite in float, you are in the right ground. Typical habit: fibrous, radial, botryoidal, or acicular crystals. Train your eye for that shape before scanning the outcrop.
Common questions
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Find frondelite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
