Cranswickite is a rare hydrated magnesium sulfate that typically occurs as a secondary mineral in oxidized sulfide deposits. Collectors should look for delicate white fibrous crusts, which are highly sensitive to humidity and must be kept in sealed containers to prevent dehydration.
Is this cranswickite?
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 cranswickite with a known reference. Cranswickite 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. Cranswickite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Cranswickite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, colorless.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: fibrous, efflorescent crusts.
Often confused with
Cranswickite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside cranswickite
Minerals reported to co-occur with cranswickite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- MgSO₄·4H₂O
- Mohs hardness
- 2
- Density
- 1.89 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous, Efflorescent Crusts
- Cleavage
- Perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Oxidized Ore Deposits
- Typical price
- $20-100 for rare micro-specimens
Where rockhounds find cranswickite
Classic worldwide localities
- Cranswick Bay, Canada
- Chuquicamata Mine, Chile
Field-hunting tip
Look in oxidized ore deposits country — that is the host setting where cranswickite typically forms. If you start seeing epsomite, hexahydrite, gypsum in float, you are in the right ground. Field specimens usually show a fibrous, efflorescent crusts habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find cranswickite on the map
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