Thaumasite

Thaumasite

Thaumasite (white) with prehnite (green) from Fairfax Quarry, Virginia
General
Category Sulfate minerals
Formula
(repeating unit)

Ca3Si(OH)6(CO3)(SO4)·12H2O
CaSi(OH)6·CaCO3·CaSO4·12H2O

CaSiO3·CaCO3·CaSO4·15H2O
Strunz classification 7.DG.15
Crystal system Hexagonal
Crystal class Pyramidal (6)
H-M symbol: (6)
Space group P63
Unit cell a = 11.030(7), c = 10.396(6) [Å]; Z = 2
Identification
Formula mass 622.62 g/mol
Color Colorless, white, pale yellow
Crystal habit Prismatic, fibrous, massive, radial
Cleavage Indistinct
Fracture Subconchoidal
Tenacity Brittle
Mohs scale hardness 3.5
Luster Vitreous to silky
Streak White
Diaphaneity Transparent to translucent
Specific gravity 1.877
Optical properties Uniaxial (-)
Refractive index nω = 1.498–1.507 nε = 1.458–1.470
Birefringence δ = 0.039
References [1][2][3]

Thaumasite is a silicate mineral with chemical formula Ca3Si(OH)6(CO3)(SO4)·12H2O. 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.

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.[4][5] 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 also

References

  1. 1 2 Mineral Handbook
  2. ↑ Webmineral data
  3. 1 2 Mindat.org
  4. ↑ 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.
  5. ↑ 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 reading

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. Retrieved 2010-08-03. 

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. Retrieved 2010-08-03. 

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