Bassanite is a rare calcium sulfate mineral often found as an dehydration product of gypsum in volcanic fumaroles or arid evaporite environments. It typically occurs as white fibrous or powdery coatings and is structurally related to the industrial material known as plaster of Paris.
Is this bassanite?
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 bassanite with a known reference. Bassanite sits at Mohs 3, 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: earthy.
- 4Match the color rangeCompare against the expected color range: colorless, snow white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular, microscopic crystals.
Often confused with
Bassanite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Bassanite is noticeably harder (Mohs 3 vs. 1.5-2); luster reads earthy on Bassanite and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.
How to tell apart: Luster reads earthy on Bassanite and pearly on {010}, vitreous to greasy on {001}; vitreous on {100} on Anhydrite.
Often found alongside bassanite
Minerals reported to co-occur with bassanite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaSO₄·0.5H₂O
- Mohs hardness
- 3
- Density
- 2.69-2.76 g/cm³
- Colors
- Streak
- White
- Luster
- Earthy
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular, microscopic crystals
- Cleavage
- Good
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- Altered from gypsum in leucite tephrite blocks; fumarolic (Vesuvius, Italy); in dry or perennially dry lake beds (California, Australia); in caves, interlayered with gypsum
Where rockhounds find bassanite
1 mapped spotsClassic worldwide localities
- Caves in Big Bend National Park, Brewster Co., Texas, USA
- Vesuvius, Campania, Italy
- Wadi Mestaoua, east of Foum Tataouine, Tunisia
- South Victoria Land, Antarctica
Field-hunting tip
Where it forms: Altered from gypsum in leucite tephrite blocks; fumarolic (Vesuvius, Italy); in dry or perennially dry lake beds (California, Australia); in caves, interlayered with gypsum. If you start seeing gypsum, anhydrite, celestine in float, you are in the right ground. Typical habit: acicular, microscopic crystals. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah. Start trip planning there.
Common questions
How do you identify bassanite?+
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Can I find bassanite in the United States?+
What rocks look like bassanite?+
What minerals are found with bassanite?+
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Find bassanite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
