Hydroniumjarosite is a member of the alunite supergroup typically found as fine-grained, powdery, or crust-like coatings in acid-rich environments. It forms as a secondary mineral during the oxidation of iron-bearing sulfides and is often visually indistinguishable from other jarosite-group members without chemical analysis.
Is this hydroniumjarosite?
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 hydroniumjarosite with a known reference. Hydroniumjarosite 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. Hydroniumjarosite leaves a light yellow streak.
- 3Read the lusterHold the specimen under a strong light. Hydroniumjarosite typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: yellow, brownish-yellow, ochre-yellow.
- 5Look at form & habitCrystal system: trigonal. Typical habit: powdery, earthy, crusts, minute rhombohedral crystals.
Often confused with
Hydroniumjarosite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Hydroniumjarosite leaves light yellow, Jarosite leaves yellow; luster reads dull on Hydroniumjarosite and vitreous on Jarosite.
How to tell apart: Streak differs — Hydroniumjarosite leaves light yellow, Natrojarosite leaves yellow.
How to tell apart: Iron Ore is the harder of the two (Mohs 5-6.5 vs. 2.5-3.5); streak differs — Hydroniumjarosite leaves light yellow, Iron Ore leaves reddish-brown to black; luster reads dull on Hydroniumjarosite and metallic to submetallic on Iron Ore.
Often found alongside hydroniumjarosite
Minerals reported to co-occur with hydroniumjarosite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (H₃O)Fe₃(SO₄)₂(OH)₆
- Mohs hardness
- 2.5-3.5
- Density
- 2.9-3.0 g/cm³
- Streak
- Light Yellow
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Powdery, Earthy, Crusts, Minute Rhombohedral Crystals
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- Acid Mine Drainage Environments, Oxidized Zones of Sulfide Ore Deposits
- Typical price
- $10-50 for small specimens
Where rockhounds find hydroniumjarosite
Classic worldwide localities
- Rio Tinto, Spain
- Iron Mountain, California, USA
- Cerro Gordo, California, USA
Field-hunting tip
Look in acid mine drainage environments, oxidized zones of sulfide ore deposits country — that is the host setting where hydroniumjarosite typically forms. If you start seeing jarosite, gypsum, goethite in float, you are in the right ground. Field specimens usually show a powdery, earthy, crusts, minute rhombohedral crystals habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find hydroniumjarosite on the map
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