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, 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
How do you identify alunite?+
Where is alunite found?+
Can I find alunite in the United States?+
How much is alunite worth?+
What rocks look like alunite?+
What minerals are found with alunite?+
What kind of rock does alunite form in?+
What is alunite used for?+
Find alunite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
