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Acmonidesite is a very rare ammonium iron sulfate chloride mineral found primarily in volcanic fumaroles. It typically forms as small, brown 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.

Hardness
3
Mohs
Luster
Vitreous
Streak
Light brown
Transparency
Transparent

Is this acmonidesite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

  • 1
    Test the hardness
    Try to scratch acmonidesite with a known reference. Acmonidesite sits at Mohs 3, softer than the next harder reference, harder than the previous one.
  • 2
    Check the streak
    Drag the specimen across an unglazed porcelain plate. Streak: light brown.
  • 3
    Read the luster
    Hold the specimen under a strong light. Luster: vitreous.
  • 4
    Match the color range
    Compare against the expected color range: brown.
  • 5
    Look at form & habit
    Crystal 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.

Kremersite mineral
mineral
Kremersite
Mohs 2

How to tell apart: Acmonidesite is noticeably harder (Mohs 3 vs. 2); streak differs, so Acmonidesite leaves light brown, Kremersite leaves yellowish.

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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)₈PbFe₄(SO₄)₅Cl₈
Mohs hardness
3
Density
2.56 g/cm³
Colors
Brown
Streak
Light brown
Luster
Vitreous
Transparency
Transparent
Crystal system
Orthorhombic
Crystal habit
Tabular crystals, crusts
Cleavage
None
Rarity
Rare
Uses
Collector
Host rock
A fumarolic sublimate on pyroclastic breccia

Where rockhounds find acmonidesite

Classic worldwide localities

  • La Fossa crater, Vulcano, Aeolian Islands, Italy

Field-hunting tip

Where it forms: A fumarolic sublimate on pyroclastic breccia. If you start seeing salammoniac, alunite, adranosite in float, you are in the right ground. Typical habit: tabular crystals, crusts. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify acmonidesite?+
Mohs hardness is 3. Luster: vitreous. The streak is light brown. Common colors include brown.
Where is acmonidesite found?+
Notable localities include La Fossa crater, Vulcano, Aeolian Islands, Italy.
Is acmonidesite safe to handle?+
Acmonidesite contains lead. Wash your hands afterwards, never grind or breathe the dust, and store it apart from specimens you handle often.
What rocks look like acmonidesite?+
Acmonidesite is most often confused with Kremersite. A quick hardness test and a streak check separate the look-alikes faster than color alone.
What minerals are found with acmonidesite?+
Acmonidesite commonly co-occurs with Salammoniac, Alunite, Adranosite. Spotting any of these in float or country rock is a useful trip signal.
What kind of rock does acmonidesite form in?+
Where it forms: A fumarolic sublimate on pyroclastic breccia. Working float back to the host body is the standard way to chase a fresh occurrence.
What is acmonidesite used for?+
Acmonidesite is used in collector.

Find acmonidesite on the map

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