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 3.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Kidwellite leaves a yellowish-white streak.
- 3Read the lusterHold the specimen under a strong light. Kidwellite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: yellow, yellow-green, brownish-yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: bladed crystals, radial aggregates, coatings.
Often confused with
Kidwellite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Rockbridgeite is the harder of the two (Mohs 4.5 vs. 3.5); streak differs — Kidwellite leaves yellowish-white, Rockbridgeite leaves greenish-brown.
How to tell apart: Streak differs — Kidwellite leaves yellowish-white, Dufrénite leaves light green.
How to tell apart: Streak differs — Kidwellite leaves yellowish-white, Strengite leaves white.
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
- 3.5
- Density
- 2.8 g/cm³
- Streak
- Yellowish-white
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Bladed Crystals, Radial Aggregates, Coatings
- Cleavage
- Perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Phosphate-rich Granite Pegmatites
- Typical price
- $20-150 per specimen
Where rockhounds find kidwellite
Classic worldwide localities
- Tip Top Mine, South Dakota, USA
- Cornwall, England
- Hagendorf, Germany
Field-hunting tip
Look in phosphate-rich granite pegmatites country — that is the host setting where kidwellite typically forms. If you start seeing rockbridgeite, frondelite, leucophosphite in float, you are in the right ground. Field specimens usually show a bladed crystals, radial aggregates, coatings habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find kidwellite on the map
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