Alunogen is a hydrated aluminum sulfate mineral that typically forms as a white, fibrous, or powdery efflorescence in mining environments or volcanic fumaroles. It is extremely soluble and fragile, requiring careful handling and storage in a dry environment to prevent dehydration or dissolution.
Is this alunogen?
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 alunogen with a known reference. Alunogen sits at Mohs 1.5-2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to silky.
- 4Match the color rangeCompare against the expected color range: white, yellowish white, reddish white.
- 5Look at form & habitCrystal system: triclinic. Typical habit: acicular, encrustations, fibrous.
Often confused with
Alunogen vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside alunogen
Minerals reported to co-occur with alunogen. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₂(SO₄)₃·17H₂O
- Mohs hardness
- 1.5-2
- Density
- 1.72-1.77 g/cm³
- Streak
- White
- Luster
- Vitreous to silky
- Transparency
- Transparent
- Crystal system
- Triclinic
- Crystal habit
- Acicular, encrustations, fibrous
- Cleavage
- {010}, perfect; probable on {100} and {313}
- Rarity
- Common
- Uses
- Collector
- Host rock
- Forms by reaction of sulfates from decomposing sulfides with aluminous minerals in shales and slates; in gossan or altered wall rock of pyritic deposits in arid regions; in coal seams
Where rockhounds find alunogen
2 mapped spotsClassic worldwide localities
- Smoky Mountain, Jackson Co., North Carolina, USA
- Alum Cave Bluff, Sevier Co., Tennessee, USA
- Alum Mountain district, Grant Co., New Mexico, USA
- Geysers, Sonoma Co., California, USA
- Nová Baňa (Königsberg), Slovakia
- Vesuvius, and from Solfatara di Pozzuoli, Campi Flegrei, and at Miseno, near Naples, Campania, Italy
Field-hunting tip
Where it forms: Forms by reaction of sulfates from decomposing sulfides with aluminous minerals in shales and slates; in gossan or altered wall rock of pyritic deposits in arid regions; in coal seams. If you start seeing pyrite, marcasite, halotrichite in float, you are in the right ground. Typical habit: acicular, encrustations, fibrous. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Missouri. Start trip planning there.
Common questions
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What rocks look like alunogen?+
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Find alunogen on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
