Boussingaultite is a rare ammonium magnesium sulfate that typically forms as a white, crusty efflorescence in geothermal regions or burning coal waste piles. Collectors usually find it as delicate coatings or powdery aggregates, as it is highly soluble in water and unstable in high humidity. Careful storage in a dry, airtight environment is essential to prevent degradation.
Is this boussingaultite?
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 boussingaultite with a known reference. Boussingaultite sits at Mohs 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: silky.
- 4Match the color rangeCompare against the expected color range: colorless, yellow pink, light yellow, pink.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: crusts, efflorescences, granular, sometimes small prismatic crystals.
Often confused with
Boussingaultite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads silky on Boussingaultite and vitreous on Pickeringite.
How to tell apart: Luster reads silky on Boussingaultite and vitreous on Halotrichite.
How to tell apart: Luster reads silky on Boussingaultite and vitreous, silky in fibrous aggregates on Epsomite.
Often found alongside boussingaultite
Minerals reported to co-occur with boussingaultite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (NH₄)₂Mg(SO₄)₂·6H₂O
- Mohs hardness
- 2
- Density
- 1.722 g/cm³
- Colors
- Streak
- White
- Luster
- Silky
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Crusts, efflorescences, granular, sometimes small prismatic crystals
- Cleavage
- {201}, perfect (synthetic)
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- A rare sublimate formed under fumarolic conditions
Where rockhounds find boussingaultite
Classic worldwide localities
- South Mountain, near Santa Paula, Ventura Co., California, USA
- Substantial quantity in Geyser Creek Canyon, The Geysers, Sonoma Co., California, USA
- Coso Hot Springs, Inyo Co., California, USA
- Travale, Val di Cecina, and Larderello, Tuscany, Italy
- Schoeller mine, Libušin, near Kladno, Czech Republic
- Gcwihaba Cave, 280 km west of Maun, northwestern Botswana
Field-hunting tip
Where it forms: A rare sublimate formed under fumarolic conditions. If you start seeing alunogen, tschermigite, gypsum in float, you are in the right ground. Typical habit: crusts, efflorescences, granular, sometimes small prismatic crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify boussingaultite?+
Where is boussingaultite found?+
What rocks look like boussingaultite?+
What minerals are found with boussingaultite?+
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What is boussingaultite used for?+
Find boussingaultite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
