Acmonidesite is a very rare ammonium iron sulfate chloride mineral found primarily in volcanic fumaroles. It typically forms as small, bright yellow tabular crystals or crusts deposited by volcanic gases at the La Fossa crater in Italy. Due to its sensitivity to humidity and unstable nature, specimens should be stored in a dry, sealed container.
Is this acmonidesite?
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 acmonidesite with a known reference. Acmonidesite sits at Mohs 3 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Acmonidesite leaves a yellowish-white streak.
- 3Read the lusterHold the specimen under a strong light. Acmonidesite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: yellow, orange-yellow.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: tabular crystals, crusts.
Often confused with
Acmonidesite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside acmonidesite
Minerals reported to co-occur with acmonidesite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (NH₄)₄Fe₂(SO₄)₃Cl₂
- Mohs hardness
- 3
- Density
- 3.23 g/cm³
- Colors
- Streak
- Yellowish-white
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular Crystals, Crusts
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Fumarolic Deposits
- Typical price
- $50-300 per specimen
Where rockhounds find acmonidesite
Classic worldwide localities
- La Fossa crater, Vulcano, Aeolian Islands, Italy
Field-hunting tip
Look in fumarolic deposits country — that is the host setting where acmonidesite typically forms. If you start seeing sulphur, salmiac, kremersite in float, you are in the right ground. Field specimens usually show a tabular crystals, crusts habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find acmonidesite on the map
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