Thaumasite is a calcium silicate mineral, containing Si atoms in unusual octahedral configuration, with chemical formula Ca3Si(OH)6(CO3)(SO4)·12H2O, also sometimes more simply written as CaSiO3·CaCO3·CaSO4·15H2O.

Thaumasite w-prehnite Hydrous basic calcium carbonate sulfate and silicate Fairfax Quarry near Contraville Fairfax County Virginia 1641.jpg
Thaumasite (white) with prehnite (green) from Fairfax Quarry, Virginia
CategorySulfate minerals
(repeating unit)


Strunz classification7.DG.15
Crystal systemHexagonal
Crystal classPyramidal (6)
H-M symbol: (6)
Space groupP63
Unit cella = 11.030(7), c = 10.396(6) [Å]; Z = 2
Formula mass622.62 g/mol
ColorColorless, white, pale yellow
Crystal habitPrismatic, fibrous, massive, radial
Mohs scale hardness3.5
LusterVitreous to silky
DiaphaneityTransparent to translucent
Specific gravity1.877
Optical propertiesUniaxial (-)
Refractive indexnω = 1.498–1.507 nε = 1.458–1.470
Birefringenceδ = 0.039

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.

Group of thaumasite prisms from the Kuruman, Kalahari manganese fields, Northern Cape Province, South Africa (size: 4.2 x 3.1 x 1.2 cm)

It occurs as a hydrothermal alteration mineral in sulfide ore deposits and geothermal alteration of basalt and tuff. It occurs with zeolites, apophyllite, analcime, calcite, gypsum and pyrite.[1]

Thaumasite can also be formed along with other calcium silicate hydrates (CSH) during cement alteration, especially when sulfate attack develops. The reaction can lead to softening, expansion and cracking of concrete. Unlike conventional sulfate attack, in which the calcium hydroxide and calcium aluminate hydrates react with sulfates to form gypsum and ettringite, in the case of thaumasite formation the calcium silicate hydrates in the cement paste can also be attacked. As a consequence, even concrete containing sulphate-resisting Portland cement may be affected.[4]

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.[3]

The silicate structure of thaumasite is unusual due to the presence of non-tetrahedral silicon in its crystal lattice.[5][6] Indeed, an atypic octahedral configuration is observed for Si present in thaumasite in the form of hexahydroxysilicate: [Si(OH)6]2−, a species exhibiting a geometry similar to that of the hexafluorosilicate [SiF6]2−.

See alsoEdit

Other calcium silicate hydrate (C-S-H) minerals:


  1. ^ a b Mineral Handbook
  2. ^ Webmineral data
  3. ^ a b
  4. ^ Wimpeny, D. E.; D. Slater; Ravindra K. Dhir; M. Roderick Jones; Li Zheng (2015-07-07). "Thaumasite in concrete structures: Some UK case studies". Challenges of Concrete Construction: Volume 3: 127–137. doi:10.1680/rraeoc.31753.0014.
  5. ^ Duffy, J. A.; D. E. Macphee (2007). "The coordination number of silicon in silicon−oxygen compounds: The special case of 6-fold coordination in thaumasite". The Journal of Physical Chemistry B. 111 (30): 8740–8745. doi:10.1021/jp071343n. PMID 17608517.
  6. ^ Jacobsen, S. D.; J. R. Smyth; R. J. Swope (2003-07-01). "Thermal expansion of hydrated six-coordinate silicon in thaumasite, Ca3Si(OH)6(CO3)(SO4)·12H2O". Physics and Chemistry of Minerals. 30 (6): 321–329. Bibcode:2003PCM....30..321J. doi:10.1007/s00269-003-0328-0.

Further readingEdit

Aguilera, J.; S. Martínez-Ramírez; I. Pajares-Colomo; M. T. Blanco-Varela (December 2003). "Formation of thaumasite in carbonated mortars". Cement and Concrete Composites. 25 (8): 991–996. doi:10.1016/S0958-9465(03)00121-5. ISSN 0958-9465.

Barnett, S. J.; C. D. Adam; A. R. W. Jackson (2000). "Solid solutions between ettringite, Ca6Al2(SO4)3(OH)12·26H2O, and thaumasite, Ca3SiSO4CO3(OH)6·12H2O". Journal of Materials Science. 35 (16): 4109–4114. Bibcode:2000JMatS..35.4109B. doi:10.1023/A:1004898623884.

Barnett, S. J.; D. E. Macphee; E. E. Lachowski; N. J. Crammond (May 2002). "XRD, EDX and IR analysis of solid solutions between thaumasite and ettringite". Cement and Concrete Research. 32 (5): 719–730. doi:10.1016/S0008-8846(01)00750-5. ISSN 0008-8846.

Matschei, Thomas; Glasser, Fredrik P. (2014). "Thermal stability of thaumasite". Materials and Structures. 48 (7): 2277–2289. doi:10.1617/s11527-014-0309-4. ISSN 1359-5997.

Rahman, M.M.; Bassuoni, M.T. (2014). "Thaumasite sulfate attack on concrete: Mechanisms, influential factors and mitigation". Construction and Building Materials. 73: 652–662. doi:10.1016/j.conbuildmat.2014.09.034. ISSN 0950-0618.

External linksEdit