Thaumasite

Thaumasite
Thaumasite (white) with prehnite (green) from Fairfax Quarry, Virginia
General
Category	Sulfate minerals
Chemical formula	Ca3Si(OH)6(CO3)(SO4)·12H2O CaSi(OH)6·CaCO3·CaSO4·12H2O CaSiO3·CaCO3·CaSO4·15H2O
Strunz classification	07.DG.15
Crystal symmetry	Hexagonal 6
Unit cell	a = 11.030(7) Å, c = 10.396(6) Å; Z = 2
Identification
Molar mass	622.62 g/mol
Color	Colorless, white, pale yellow
Crystal habit	Prismatic, fibrous, massive, radial
Crystal system	Hexagonal dipyramidal
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]

Another quite surprising feature of thaumasite is the presence of non-tetrahedral silicon in its crystal lattice.^{[4][5][citation needed]} 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

• Calcium silicate hydrate
• Afwillite
• Jennite
• Tobermorite

References

1. ^ ^{a b} Mineral Handbook
2. ^ Webmineral data
3. ^ ^{a b} 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. doi:10.1007/s00269-003-0328-0. 

Further readings

Aguilera, J.; S. Martínez-Ramírez, I. Pajares-Colomo, M. T. Blanco-Varela (2003-12). "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. http://www.sciencedirect.com/science/article/B6TWF-4979J9S-C/2/f67125433f87c0db9e029b368c957ec1. 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. doi:10.1023/A:1004898623884. 

Barnett, S. J.; D. E. Macphee, E. E. Lachowski, N. J. Crammond (2002-05). "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. http://www.sciencedirect.com/science/article/B6TWG-44XC8CF-8/2/cdd4c383057e2d08e0e3147c980dd41b. Retrieved 2010-08-03. 

External links

• Thaumasite form of sulfate attack (TSA)