Mundrabillaite is a rare ammonium calcium phosphate mineral that typically forms as delicate, needle-like white crusts or efflorescences. It is primarily found in environments where bat guano reacts with calcium-rich substrates in caves or, famously, at the site of the Mundrabilla iron meteorite. Collectors prize it as a scientific curiosity due to its unusual association with phosphate-rich organic decay.
Is this mundrabillaite?
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 mundrabillaite with a known reference. Mundrabillaite sits at Mohs 1-2, 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: dull.
- 4Match the color rangeCompare against the expected color range: colorless.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: microscopic crystals.
Often confused with
Mundrabillaite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside mundrabillaite
Minerals reported to co-occur with mundrabillaite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (NH₄)₂Ca(HPO₄)₂·H₂O
- Mohs hardness
- 1-2
- Density
- 2.05 g/cm³
- Colors
- Streak
- White
- Luster
- Dull
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Microscopic crystals
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Derived from bat guano and urine in caves
Where rockhounds find mundrabillaite
Classic worldwide localities
- Petrogale Cave, near Madura, Western Australia
- Arnhem Cave, 150 km east of Windhoek, Namibia
Field-hunting tip
Where it forms: Derived from bat guano and urine in caves. If you start seeing biphosphammite, archerite, aphthitalite in float, you are in the right ground. Typical habit: microscopic crystals. Train your eye for that shape before scanning the outcrop.
Common questions
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Find mundrabillaite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
