Thaumasite

Thaumasite is named after Greek "thaumazein" - to be surprised, alluding to its unusual composition, which includes silicate, carbonate and sulfate radicals.

Thaumasite forms transparent to translucent white, yellowish, sometimes pink or brown acicular prismatic crystals or compact masses.

Yellow material from South Africa has been faceted.
General Information
Chemical Formula
Ca
 
6
Si
 
2
(CO
 
3
)
 
2
(SO
 
4
)
 
2
(OH)
 
12
· 24H
 
2
O
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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Photos of natural/un-cut material from mindat.org
Physical Properties of Thaumasite
Mohs Hardness3.5
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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Specific Gravity1.91
Walter Schumann, Gemstones of the world (2001)
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TenacityBrittle
Anthony et al, Handbook of mineralogy (2001)
Cleavage QualityPoor
Anthony et al, Handbook of mineralogy (2001) Traces observed
FractureSub-Conchoidal
Anthony et al, Handbook of mineralogy (2001)
Optical Properties of Thaumasite
Refractive Index1.467 to 1.505
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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Optical CharacterUniaxial/-
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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Birefringence0.038
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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Colour
Colour (General)Colourless, light yellow, white.
Walter Schumann, Gemstones of the world (2001)
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TransparencyTransparent
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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LustreVitreous
Walter Schumann, Gemstones of the world (2001)
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Crystallography of Thaumasite
Crystal SystemHexagonal
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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HabitAcicular prismatic crystals or compact masses
Michael O’Donoghue, Gems, Sixth Edition (2006)
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Geological Environment
Where found:Occurs as a very late-stage mineral in some sulphide ore deposits.
Michael O’Donoghue, Gems, Sixth Edition (2006)
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Further Information
Mineral information:Thaumasite information at mindat.org
Significant Gem Localities
South Africa
 
  • Northern Cape
    • Kalahari manganese field
      • Hotazel
Gems, Sixth Edition, Michael O’Donoghue, 2006, p. 459
      • Kuruman
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