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 3.5-4.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: light yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: dull, may be waxy to earthy.
- 4Match the color rangeCompare against the expected color range: yellow.
- 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: Streak differs, so Ammoniojarosite leaves light yellow, Jarosite leaves pale yellow; luster reads dull, may be waxy to earthy on Ammoniojarosite and subadamantine to vitreous, resinous on fractures on Jarosite.
How to tell apart: Luster reads dull, may be waxy to earthy on Ammoniojarosite and vitreous on Natrojarosite.
How to tell apart: Streak differs, so Ammoniojarosite leaves light yellow, Goethite leaves brownish yellow, yellow-orange, ocher-yellow; luster reads dull, may be waxy to earthy on Ammoniojarosite and imperfect adamantine metallic to dull earthy; silky when fibrous on Goethite.
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
- 3.5-4.5
- Density
- 3.02 g/cm³
- Colors
- Streak
- Light yellow
- Luster
- Dull, may be waxy to earthy
- Transparency
- Semitransparent
- Crystal system
- Trigonal
- Crystal habit
- Massive, crusts, earthy aggregates, microcrystalline
- Cleavage
- Distinct on {0001}
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- Formed in black lignitic shales containing pyrite (Utah, USA)
Where rockhounds find ammoniojarosite
Classic worldwide localities
- An undefined locality on the “west side of the Kaibab fault, Kane Co., Utah”, USA
- Buffalo, Johnson Co. and about 5 km south of Wamsutter, Sweetwater Co., Wyoming, USA
- Sulphur Bank mine, Lake Co. and The Geysers, Sonoma Co., California, USA
- Cebadillas, Potosí, Bolivia
- Zlatá Baňa district, Slanské Mountains, Slovakia
- Le Kaymar, Aveyron, France
Field-hunting tip
Where it forms: Formed in black lignitic shales containing pyrite (Utah, USA). If you start seeing tschermigite, epsomite, jarosite in float, you are in the right ground. Typical habit: massive, crusts, earthy aggregates, microcrystalline. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify ammoniojarosite?+
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Find ammoniojarosite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
