Pentahydrite is a rare magnesium sulfate hydrate that typically forms as a secondary efflorescence in dry environments or arid mine workings. It is notoriously unstable and will dehydrate quickly in air, often altering to hexahydrite or epsomite, making it a challenge for collectors to preserve.
Is this pentahydrite?
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 pentahydrite with a known reference. Pentahydrite 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. Pentahydrite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Pentahydrite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, colorless, pale yellow, pale green, pale blue.
- 5Look at form & habitCrystal system: triclinic. Typical habit: fibrous, crusts, efflorescent coatings.
Often confused with
Pentahydrite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside pentahydrite
Minerals reported to co-occur with pentahydrite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- MgSO₄·5H₂O
- Mohs hardness
- 2
- Density
- 1.72 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Triclinic
- Crystal habit
- Fibrous, Crusts, Efflorescent Coatings
- Cleavage
- Perfect
- Rarity
- Uncommon
- Uses
- Collector, Scientific Research
- Host rock
- Evaporite Deposits, Volcanic Fumaroles, Mine Tailings
- Typical price
- $10-50 per specimen
Where rockhounds find pentahydrite
Classic worldwide localities
- Germany
- Chile
- USA
- Italy
Field-hunting tip
Look in evaporite deposits, volcanic fumaroles, mine tailings country — that is the host setting where pentahydrite typically forms. If you start seeing epsomite, hexahydrite, gypsum in float, you are in the right ground. Field specimens usually show a fibrous, crusts, efflorescent coatings habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify pentahydrite?+
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Find pentahydrite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
