Lafossaite is a rare thallium halide, typically gray-brown. It is a by sublimation from fumarolic gas. Associated minerals include cannizzarite, galenobismutite and pyrite. It was first described from La Fossa Crater, Vulcano, Aeolian archipelago, Sicily, Italy.
Is this lafossaite?
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 lafossaite with a known reference. Lafossaite sits at Mohs 3-4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: off-white to cream.
- 3Read the lusterHold the specimen under a strong light. Luster: resinous to greasy.
- 4Match the color rangeCompare against the expected color range: grayish brown.
- 5Look at form & habitCrystal system: cubic. Typical habit: cubic crystals, subhedral crystals.
Often confused with
Lafossaite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Lafossaite leaves off-white to cream, Halite leaves white; luster reads resinous to greasy on Lafossaite and vitreous on Halite.
How to tell apart: Lafossaite is noticeably harder (Mohs 3-4 vs. 2); streak differs, so Lafossaite leaves off-white to cream, Sylvite leaves white; luster reads resinous to greasy on Lafossaite and vitreous on Sylvite.
Often found alongside lafossaite
Minerals reported to co-occur with lafossaite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- TlCl
- Mohs hardness
- 3-4
- Density
- 7.212 g/cm³
- Colors
- Grayish brown
- Streak
- Off-white to cream
- Luster
- Resinous to greasy
- Transparency
- Translucent
- Crystal system
- Cubic
- Crystal habit
- Cubic crystals, subhedral crystals
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- By sublimation from fumarolic gas
Where rockhounds find lafossaite
Classic worldwide localities
- La Fossa Crater, Vulcano, Aeolian archipelago, Sicily, Italy
Field-hunting tip
Where it forms: By sublimation from fumarolic gas. If you start seeing cannizzarite, galenobismutite, pyrite in float, you are in the right ground. Typical habit: cubic crystals, subhedral crystals. Train your eye for that shape before scanning the outcrop.
Common questions
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Find lafossaite on the map
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