Ammoniojarosite is a rare sulfate mineral that typically forms as a secondary yellow crust or earthy mass in volcanic fumaroles or burning coal seams. It is structurally related to the jarosite group, distinguished by the presence of the ammonium ion in its chemical composition. Collectors typically find it as an inconspicuous coating or powder on other secondary minerals.
Is this ammoniojarosite?
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 ammoniojarosite with a known reference. Ammoniojarosite sits at Mohs 2.5-3.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Ammoniojarosite leaves a yellow streak.
- 3Read the lusterHold the specimen under a strong light. Ammoniojarosite typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: yellow, yellow-brown, brown.
- 5Look at form & habitCrystal system: trigonal. Typical habit: massive, crusts, earthy aggregates, microcrystalline.
Often confused with
Ammoniojarosite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Luster reads dull on Ammoniojarosite and vitreous on Jarosite.
How to tell apart: Iron Ore is the harder of the two (Mohs 5-6.5 vs. 2.5-3.5); streak differs — Ammoniojarosite leaves yellow, Iron Ore leaves reddish-brown to black; luster reads dull on Ammoniojarosite and metallic to submetallic on Iron Ore.
Often found alongside ammoniojarosite
Minerals reported to co-occur with ammoniojarosite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (NH₄)Fe₃(SO₄)₂(OH)₆
- Mohs hardness
- 2.5-3.5
- Density
- 2.7-2.8 g/cm³
- Colors
- Streak
- Yellow
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Massive, Crusts, Earthy Aggregates, Microcrystalline
- Cleavage
- Distinct On {0001}
- Rarity
- Uncommon
- Uses
- Collector, Scientific Research
- Host rock
- Fumarolic Deposits, Coal Mine Dumps, Hydrothermal Environments
- Typical price
- $10-60 for small mineral specimens
Where rockhounds find ammoniojarosite
Classic worldwide localities
- Red Canyon, Utah, USA
- Cerro Pintado, Chile
- Kurnakov volcano, Iturup Island, Russia
Field-hunting tip
Look in fumarolic deposits, coal mine dumps, hydrothermal environments country — that is the host setting where ammoniojarosite typically forms. If you start seeing jarosite, gypsum, sulfur in float, you are in the right ground. Field specimens usually show a massive, crusts, earthy aggregates, microcrystalline habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find ammoniojarosite on the map
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