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. Alunite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Alunite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, gray, pink, reddish, yellow.
- 5Look at form & habitCrystal system: trigonal. Typical habit: tabular rhombohedral crystals, often massive or granular.
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
- Transparency
- Translucent
- Crystal system
- Trigonal
- Crystal habit
- Tabular Rhombohedral Crystals, Often Massive or Granular
- Cleavage
- Distinct Basal
- Rarity
- Common
- Uses
- Industrial, Collector
- Host rock
- Hydrothermal Alteration Zones in Volcanic Rocks
- Typical price
- $5-30 thumbnail, $20-100 cabinet specimen
Where rockhounds find alunite
12 mapped spotsClassic worldwide localities
- Tolfa, Italy
- Marysvale, Utah, USA
- Bujalance, Spain
- Toki, Japan
Field-hunting tip
Look in hydrothermal alteration zones in volcanic rocks country — that is the host setting where alunite typically forms. If you start seeing kaolinite, quartz, pyrite in float, you are in the right ground. Field specimens usually show a tabular rhombohedral crystals, often massive or granular habit, so 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
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Find alunite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
