Aldermanite is a very rare hydrated magnesium aluminum phosphate mineral typically found as small, fragile platy crystals or radial clusters. It is primarily known from phosphate-rich pegmatite deposits where it forms as a secondary mineral alongside other phosphate species.
Is this aldermanite?
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 aldermanite with a known reference. Aldermanite sits at Mohs 2 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Aldermanite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Aldermanite typically shows a pearly luster.
- 4Match the color rangeCompare against the expected color range: colorless, white, pale yellow.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: platy crystals, radial aggregates.
Often confused with
Aldermanite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Wavellite is the harder of the two (Mohs 3.5-4 vs. 2); luster reads pearly on Aldermanite and vitreous on Wavellite.
How to tell apart: Montgomeryite is the harder of the two (Mohs 3 vs. 2); luster reads pearly on Aldermanite and vitreous on Montgomeryite.
Often found alongside aldermanite
Minerals reported to co-occur with aldermanite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₅Al₁₀(PO₄)₄(OH)₂₀·8H₂O
- Mohs hardness
- 2
- Density
- 2.28 g/cm³
- Streak
- White
- Luster
- Pearly
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy Crystals, Radial Aggregates
- Cleavage
- Perfect On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Phosphate-rich Pegmatites
- Typical price
- $50-300 per specimen
Where rockhounds find aldermanite
Classic worldwide localities
- Big Chief mine, Keystone, South Dakota, USA
Field-hunting tip
Look in phosphate-rich pegmatites country — that is the host setting where aldermanite typically forms. If you start seeing montgomeryite, overite, wavellite in float, you are in the right ground. Field specimens usually show a platy crystals, radial aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find aldermanite on the map
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