Mirabilite is a hydrous sodium sulfate mineral that typically forms as white, crusty, or needle-like efflorescences in arid environments. It is notoriously unstable in open air, as it dehydrates rapidly to powdery thenardite when exposed to low humidity or warmth, so it must be stored in airtight containers.
Is this mirabilite?
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 mirabilite with a known reference. Mirabilite sits at Mohs 1.5-2.5, 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, colorless, yellowish white, greenish white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: crystalline, efflorescences, granular.
Often confused with
Mirabilite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside mirabilite
Minerals reported to co-occur with mirabilite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Na₂SO₄·10H₂O
- Mohs hardness
- 1.5-2.5
- Density
- 1.464 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to opaque
- Crystal system
- Monoclinic
- Crystal habit
- Crystalline, efflorescences, granular
- Cleavage
- On {100}, perfect; on {010} and {001}, good to fair
- Rarity
- Common
- Uses
- Collector, Scientific study
- Host rock
- Typically in salt pans, playas, and saline lakes, where deposition may be seasonal, and bedded deposits formed therefrom; rarely in caves and lava tubes; in volcanic fumaroles
Where rockhounds find mirabilite
1 mapped spotsClassic worldwide localities
- Soda Lake, San Luis Obispo Co., and in Searles Lake, San Bernardino Co., California, USA
- Great Salt Lake, Davis Co., Utah, USA
- Lake Lucero, west of Valmont, Doña Ana Co., New Mexico, USA
- Downey and Union Pacific Lakes, Albany Co., Wyoming, USA
- Hallein, Hallstatt, and Ischl, Austria
- Vesuvius, and at Campi Flegrei, near Naples, Campania, Italy
Field-hunting tip
Where it forms: Typically in salt pans, playas, and saline lakes, where deposition may be seasonal, and bedded deposits formed therefrom; rarely in caves and lava tubes; in volcanic fumaroles. If you start seeing gypsum, thénardite, halite in float, you are in the right ground. Typical habit: crystalline, efflorescences, granular. 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 mirabilite?+
Where is mirabilite found?+
Can I find mirabilite in the United States?+
What rocks look like mirabilite?+
What minerals are found with mirabilite?+
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What is mirabilite used for?+
Find mirabilite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
