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 2.5, 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, light blue, light greenish blue.
- 5Look at form & habitCrystal system: triclinic. Typical habit: efflorescences, granular.
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
- 2.5
- Density
- 1.896 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Triclinic
- Crystal habit
- Efflorescences, granular
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- An efflorescence on shales (Grand Junction, Colorado, USA); an efflorescence on mine timbers (Virginia City, Nevada, USA); in acid sulfate soils (Basse-Casamance Valley, Senegal)
Where rockhounds find pentahydrite
Classic worldwide localities
- Cripple Creek, Teller Co., and northwest of Grand Junction, in the West Salt Creek watershed, Mesa Co., Colorado, USA
- Geysers, Sonoma Co., California, USA
- Virginia City, Comstock district, Storey Co., Nevada, USA
- Copaquire, Tarapacá, Chile
- Basse-Casamance Valley, Senegal
Field-hunting tip
Where it forms: An efflorescence on shales (Grand Junction, Colorado, USA); an efflorescence on mine timbers (Virginia City, Nevada, USA); in acid sulfate soils (Basse-Casamance Valley, Senegal). If you start seeing alunogen, chalcanthite in float, you are in the right ground. Typical habit: efflorescences, granular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify pentahydrite?+
Where is pentahydrite found?+
What rocks look like pentahydrite?+
What minerals are found with pentahydrite?+
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What is pentahydrite used for?+
Find pentahydrite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
