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

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

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.

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.

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.