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, 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?+
Where is pentahydrite found?+
How much is pentahydrite worth?+
What rocks look like pentahydrite?+
What minerals are found with pentahydrite?+
What kind of rock does pentahydrite form in?+
What is pentahydrite used for?+
Find pentahydrite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
