Epsomite is a highly soluble hydrated magnesium sulfate mineral that typically forms as delicate, needle-like crystals or white crusts. It is most commonly found as efflorescence in caves, mine tunnels, or near volcanic vents where water evaporates quickly. Due to its extreme solubility, specimens must be stored in airtight containers to prevent dehydration and crumbling.
Is this epsomite?
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 epsomite with a known reference. Epsomite sits at Mohs 2-2.5, 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, silky in fibrous aggregates.
- 4Match the color rangeCompare against the expected color range: white, colorless, yellowish white, greenish white, pinkish white.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: acicular, encrustations, fibrous.
Often confused with
Epsomite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside epsomite
Minerals reported to co-occur with epsomite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- MgSO₄·7H₂O
- Mohs hardness
- 2-2.5
- Density
- 1.677 g/cm³
- Streak
- White
- Luster
- Vitreous, silky in fibrous aggregates
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Acicular, encrustations, fibrous
- Cleavage
- {010}, perfect; {101}, distinct
- Rarity
- Common
- Uses
- Collector, Industrial, Bath salts
- Host rock
- As efflorescences on the walls of mines, caves, and outcrops of sulfide-bearing magnesian rocks; a product of evaporation at mineral springs and saline lakes; a hydration product of kieserite and langbeinite
Where rockhounds find epsomite
2 mapped spotsClassic worldwide localities
- Mammoth Cave, Kentucky, USA
- Mt. Princeton hot spring, Chalk Creek, Chaffee Co., Colorado, USA
- Leona Heights, Alameda Co., California, USA
- Very salt lakes on Kruger Mountain, near Oroville, Okanogan Co., Washington, USA
- Epsom, Surrey, England
- Sedlitz and Saidschitz, Czech Republic
Field-hunting tip
Where it forms: As efflorescences on the walls of mines, caves, and outcrops of sulfide-bearing magnesian rocks; a product of evaporation at mineral springs and saline lakes; a hydration product of kieserite and langbeinite. If you start seeing melanterite, gypsum, halotrichite in float, you are in the right ground. Typical habit: acicular, encrustations, fibrous. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in New Mexico, Utah. Start trip planning there.
Common questions
How do you identify epsomite?+
Where is epsomite found?+
Can I find epsomite in the United States?+
What rocks look like epsomite?+
What minerals are found with epsomite?+
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Find epsomite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
