Thaumasite is a rare hydrated calcium silicate sulfate carbonate known for its unusual chemical composition. It typically forms delicate needle-like or prismatic crystals in vugs and cavities, often associated with other calcium-rich minerals. Collectors value it for its complex crystal habits and its tendency to be found in altered basalt or metamorphic environments.
Is this thaumasite?
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 thaumasite with a known reference. Thaumasite sits at Mohs 3.5, 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 to dull, greasy, silky.
- 4Match the color rangeCompare against the expected color range: white, colorless.
- 5Look at form & habitCrystal system: hexagonal. Typical habit: fibrous, massive, radial.
Often confused with
Thaumasite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Thaumasite is noticeably harder (Mohs 3.5 vs. 1.5-2); luster reads vitreous to dull, greasy, silky on Thaumasite and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.
How to tell apart: Thaumasite is noticeably harder (Mohs 3.5 vs. 2-2.5).
Often found alongside thaumasite
Minerals reported to co-occur with thaumasite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₃Si(OH)₆(CO₃)(SO₄)·12H₂O
- Mohs hardness
- 3.5
- Density
- 1.877 g/cm³
- Streak
- White
- Luster
- Vitreous to dull, greasy, silky
- Transparency
- Transparent to translucent
- Crystal system
- Hexagonal
- Crystal habit
- Fibrous, massive, radial
- Cleavage
- Traces observed
- Fluorescence
- Dull uneven white under LW UV
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- A very late-stage mineral in some sulfide ore deposits
Where rockhounds find thaumasite
Classic worldwide localities
- Paterson and Great Notch, Passaic Co., New Jersey, USA
- Goose Creek quarry, Leesburg, Loudoun Co., and the Fairfax quarry, Centreville, Fairfax Co., Virginia, USA
- Crestmore, Riverside Co., California, USA
- Bjelke mine, near Åreskuta, and at Kjölland, Jämtland, and at Långban, Värmland, Sweden
- Sulitjelma, Norway
- Ballyalton, Co. Down, Ireland
Field-hunting tip
Where it forms: A very late-stage mineral in some sulfide ore deposits. If you start seeing zeolites, apophyllite, analcime in float, you are in the right ground. Typical habit: fibrous, massive, radial. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify thaumasite?+
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What rocks look like thaumasite?+
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Find thaumasite on the map
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