Kidwellite is a rare secondary phosphate mineral that typically forms bright yellow to yellow-green radial sprays or velvety crusts. It is most frequently found in complex phosphate-rich granite pegmatites as an alteration product of primary minerals like triphylite.
Is this kidwellite?
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 kidwellite with a known reference. Kidwellite 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: yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: silky.
- 4Match the color rangeCompare against the expected color range: yellow, greenish yellow, greenish white, light green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: columnar, fibrous.
Often confused with
Kidwellite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Kidwellite leaves yellow, Rockbridgeite leaves greenish gray; luster reads silky on Kidwellite and vitreous to dull on Rockbridgeite.
How to tell apart: Streak differs, so Kidwellite leaves yellow, Dufrénite leaves green; luster reads silky on Kidwellite and vitreous to silky on Dufrénite.
How to tell apart: Streak differs, so Kidwellite leaves yellow, Strengite leaves white; luster reads silky on Kidwellite and vitreous on Strengite.
Often found alongside kidwellite
Minerals reported to co-occur with kidwellite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaFe³⁺₉(PO₄)₆(OH)₁₀·5H₂O
- Mohs hardness
- 3
- Density
- 3.04 g/cm³
- Streak
- Yellow
- Luster
- Silky
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Columnar, fibrous
- Cleavage
- Perfect on {100}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- As a late-stage replacement of earlier phosphate minerals
Where rockhounds find kidwellite
Classic worldwide localities
- Coon Creek and York mines, Polk Co, Arkansas, USA
- Fodderstack Mountain, Montgomery Co., Arkansas, USA
- Buckeye Mountain, at Three Oak Gap, Arkansas, USA
- Indian Mountain, Cherokee Co., Alabama, USA
- Rotläufchen mine, Waldgirmes, near Giessen, and the Eleonore mine, Hesse, Germany
- Clara mine, near Oberwolfach, Black Forest, Germany
Field-hunting tip
Where it forms: As a late-stage replacement of earlier phosphate minerals. If you start seeing rockbridgeite, beraunite, strengite in float, you are in the right ground. Typical habit: columnar, fibrous. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify kidwellite?+
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What rocks look like kidwellite?+
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Find kidwellite on the map
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