Ammonioalunite is a rare sulfate mineral found primarily in geothermal environments where ammonia-rich fluids interact with aluminous rocks. It typically appears as white, earthy, or powdery crusts associated with fumaroles and solfataras. Collectors primarily seek this mineral for its distinct chemical composition within the alunite supergroup.
Is this ammonioalunite?
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 ammonioalunite with a known reference. Ammonioalunite sits at Mohs 2-3, 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.
- 4Match the color rangeCompare against the expected color range: white, gray white.
- 5Look at form & habitCrystal system: trigonal. Typical habit: efflorescences, massive, rhombohedral crystals.
Often confused with
Ammonioalunite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside ammonioalunite
Minerals reported to co-occur with ammonioalunite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (NH₄)Al₃(SO₄)₂(OH)₆
- Mohs hardness
- 2-3
- Density
- 2.4 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Trigonal
- Crystal habit
- Efflorescences, massive, rhombohedral crystals
- Cleavage
- On [{0001}, perfect.] (by analogy with the alunite group)
- Rarity
- Rare
- Uses
- Collector, Research
- Host rock
- Formed in hot springs under very acid conditions, rich in ammonium and sulfate, poor in potassium, below 100°C
Where rockhounds find ammonioalunite
Classic worldwide localities
- Geysers, Sonoma Co., California, USA
Field-hunting tip
Where it forms: Formed in hot springs under very acid conditions, rich in ammonium and sulfate, poor in potassium, below 100°C. If you start seeing ammoniojarosite in float, you are in the right ground. Typical habit: efflorescences, massive, rhombohedral crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify ammonioalunite?+
Where is ammonioalunite found?+
What rocks look like ammonioalunite?+
What minerals are found with ammonioalunite?+
What kind of rock does ammonioalunite form in?+
What is ammonioalunite used for?+
Find ammonioalunite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
