Francoanellite is a rare phosphate mineral typically found as a secondary precipitate in cave guano deposits. It is often identified as soft, white, platy aggregates or crusts that form through the reaction of bat guano with aluminous bedrock or clay.
Is this francoanellite?
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 francoanellite with a known reference. Francoanellite 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: pearly.
- 4Match the color rangeCompare against the expected color range: yellowish white.
- 5Look at form & habitCrystal system: trigonal. Typical habit: nodular, pulverulent.
Often confused with
Francoanellite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside francoanellite
Minerals reported to co-occur with francoanellite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- K₃Al₅(PO₃OH)₆(PO₄)₂·12H₂O
- Mohs hardness
- 1-2
- Density
- 2.26 g/cm³
- Colors
- Streak
- White
- Luster
- Pearly
- Transparency
- Semitransparent
- Crystal system
- Trigonal
- Crystal habit
- Nodular, pulverulent
- Cleavage
- Perfect basal
- Rarity
- Rare
- Uses
- Collector
- Host rock
- An alteration product probably formed by reaction between bat guano and clay minerals (Castellana Cave, Italy)
Where rockhounds find francoanellite
Classic worldwide localities
- Above Rabbit Cave, Farallon Islands, about 48 km west of San Francisco Co., California, USA
- Puglia, from the Castellana Cave, about 40 km southeast of Bari, and in the Little Swallow Cave, near Polignaro a mare, Italy
- Paddy’s River mine, Australian Capital Territory, Australia
Field-hunting tip
Where it forms: An alteration product probably formed by reaction between bat guano and clay minerals (Castellana Cave, Italy). If you start seeing taranakite, hydroxylapatite, brushite in float, you are in the right ground. Typical habit: nodular, pulverulent. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify francoanellite?+
Where is francoanellite found?+
What rocks look like francoanellite?+
What minerals are found with francoanellite?+
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Find francoanellite on the map
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