is a silicate mineral
with an unusual composition, Ca3Si
). 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.