Alunite is a sulfate mineral that typically forms in hydrothermal settings as an alteration product of volcanic rocks. Collectors should look for its characteristic pseudo-cubic rhombohedral crystals or massive, chalk-like deposits found in geothermal areas.
Is this alunite?
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 alunite with a known reference. Alunite sits at Mohs 3.5-4, 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, somewhat pearly on {0001}.
- 4Match the color rangeCompare against the expected color range: white, gray, yellowish gray, reddish gray, yellowish white.
- 5Look at form & habitCrystal system: trigonal. Typical habit: earthy, fibrous, massive.
Often confused with
Alunite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside alunite
Minerals reported to co-occur with alunite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KAl₃(SO₄)₂(OH)₆
- Mohs hardness
- 3.5-4
- Density
- 2.6-2.9 g/cm³
- Streak
- White
- Luster
- Vitreous, somewhat pearly on {0001}
- Transparency
- Transparent to translucent
- Crystal system
- Trigonal
- Crystal habit
- Earthy, fibrous, massive
- Cleavage
- On {0001}, perfect
- Rarity
- Common
- Uses
- Industrial, Collector
- Host rock
- Formed between 15°C and 400°C by the action of sulfate, which may be generated from pyrite or solfataric action, on aluminous rocks, commonly accompanied by kaolinitization and silicification
Where rockhounds find alunite
12 mapped spotsClassic worldwide localities
- A very Marysvale, Ohio district, Piute Co., Utah, USA
- South River, Mineral Co., Red Mountain, Hinsdale Co., in the Rosita Hills, Custer Co., and approximately 100 million t in the Calico Peak porphyry, Rico district, Dolores Co., Colorado, USA
- Meiklejohn Mountain, near Beatty, Nye Co., Nevada, USA
- Lassen Volcanic National Park, Shasta Co., California, USA
- Tolfa, 14 km east-northeast of Civitavecchia, Lazio, Italy
- Mukacheve (Muzijeva) and from Beregavo, near Mukachevo, Ukraine
Field-hunting tip
Where it forms: Formed between 15°C and 400°C by the action of sulfate, which may be generated from pyrite or solfataric action, on aluminous rocks, commonly accompanied by kaolinitization and silicification. If you start seeing kaolinite, halloysite, diaspore in float, you are in the right ground. Typical habit: earthy, fibrous, massive. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah. Start trip planning there.
Common questions
How do you identify alunite?+
Where is alunite found?+
Can I find alunite in the United States?+
What rocks look like alunite?+
What minerals are found with alunite?+
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Find alunite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
