Thaumasite is a silicate mineral with an unusual composition, Ca3Si(CO3)(SO4)(OH)6·12(H2O). It occurs as colorless to white prismatic hexagonal crystals, typically as acicular radiating groups. It also occurs as fibrous masses. Its Mohs hardness is 3.5 and it has a specific gravity of 1.88 to 1.90. Optically it is uniaxial negative with indices of refraction of nω = 1.507 and nε = 1.468.
It was first described in 1878 in Sweden and named from the Greek, "thaumazein", to be surprised, in reference to its unusual composition with carbonate, sulfate and hydroxysilicate anions.
- Mineral Handbook
- Webmineral data
- Mindat with location data
- Thaumasite form of sulfate attack (TSA)
- Sulfate (sulphate) attack