Thaumasite

Thaumasite is a calcium <a href="/facts/Silicate_mineral/WclrQW2I">silicate mineral</a>, containing Si atoms in unusual <a href="/facts/Octahedron/G38q92xW">octahedral</a> configuration, with chemical formula <a href="/facts/Calcium/hf252CC0">Ca</a>3<a href="/facts/Silicon/cgWx0hdr">Si</a>(O<a href="/facts/Hydrogen/BoYHjl0J">H</a>)6(<a href="/facts/Carbon/ukrgQexo">C</a><a href="/facts/Oxygen/acyn6o8r">O</a>3)(<a href="/facts/Sulfur/IK0amTfM">S</a>O4)·12<a href="/facts/Water/0lkXnJPK">H2O</a>, also sometimes more simply written as CaSiO3·CaCO3·CaSO4·15H2O.
It occurs as colorless to white prismatic <a href="/facts/Hexagonal_crystal_system/sSSok8wo">hexagonal</a> crystals, typically as <a href="/facts/Acicular_(crystal_habit)/PGMnp4TX">acicular</a> radiating groups. It also occurs as fibrous masses. Its <a href="/facts/Mohs_hardness/wXBMcVIm">Mohs hardness</a> is 3.5 and it has a <a href="/facts/Specific_gravity/ySquGd2x">specific gravity</a> of 1.88 to 1.90. Optically it is uniaxial negative with <a href="/facts/Index_of_refraction/yjkzvgeE">indices of refraction</a> of nω = 1.507 and nε = 1.468.

 
It occurs as a <a href="/facts/Hydrothermal_alteration/loQ477Ke">hydrothermal alteration</a> mineral in sulfide ore deposits and geothermal alteration of <a href="/facts/Basalt/uhbwGPTb">basalt</a> and <a href="/facts/Tuff/PhhRoLdJ">tuff</a>. It occurs with <a href="/facts/Zeolite/SNrXy4v2">zeolites</a>, <a href="/facts/Apophyllite/PfdNnW67">apophyllite</a>, <a href="/facts/Analcime/1F8b50YT">analcime</a>, <a href="/facts/Calcite/gKMHYQJo">calcite</a>, <a href="/facts/Gypsum/MItYU70n">gypsum</a> and <a href="/facts/Pyrite/41CYREyr">pyrite</a>.
Thaumasite can also be formed in man-made <a href="/facts/Concrete/K6whGVNx">concrete</a> structures at the detriment of <a href="/facts/Calcium_silicate_hydrate/rYewAGuv">calcium silicate hydrates</a> (C-S-H, with dashes denoting the non-<a href="/facts/Stoichiometry/I5p9cJzV">stoichiometry</a> of this hydrated cement phase acting as the "glue" in hardened cement paste) during <a href="/facts/Cement/VfDMkk12">cement</a> alteration, especially when <a href="/facts/Sulfate_attack/EfyRPGzn">sulfate attack</a> develops. The reaction consuming the silicates of the "cement glue" can lead to harmful <a href="/facts/Cohesion_(geology)/e3lj9YB5">decohesion</a> and softening (more rarely to expansion and cracking) of <a href="/facts/Concrete/K6whGVNx">concrete</a>. Unlike conventional <a href="/facts/Sulfate_attack_in_concrete_and_mortar/EfyRPGzn">sulfate attack</a>, in which the <a href="/facts/Calcium_hydroxide/Kz2KzRd8">calcium hydroxide</a> (<a href="/facts/Portlandite/vAo4Y3St">portlandite</a>) and <a href="/facts/Calcium_aluminates/YH65wHIN">calcium aluminate</a> hydrates react with <a href="/facts/Sulfate/yuWVxsUf">sulfates</a> to form <a href="/facts/Gypsum/MItYU70n">gypsum</a> and <a href="/facts/Ettringite/yN9up1Ur">ettringite</a> (an expansive phase) respectively, in the case of the thaumasite form of sulfate attack (TSA) the calcium silicate hydrates ensuring the cohesion in the hardened cement paste are also destroyed. As a consequence, even concrete containing sulfate-resisting <a href="/facts/Portland_cement/kPj8EjnP">Portland cement</a> may be affected.
It was first described in 1878 in Sweden and named from the <a href="/facts/Greek_language/pzDd6eIz">Greek</a>, "thaumazein", to be surprised, in reference to its unusual composition with <a href="/facts/Carbonate/FES1QHkN">carbonate</a>, sulfate and hydroxysilicate anions.
The silicate structure of thaumasite is unusual due to the presence of <a href="/facts/Silicate/7NdthSRM">non-tetrahedral silicon</a> in its <a href="/facts/Crystal_lattice/7gQKTsXk">crystal lattice</a>. Indeed, an atypic <a href="/facts/Octahedral/G38q92xW">octahedral</a> 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 <a href="/facts/Hexafluorosilicate/J35wRTRh">hexafluorosilicate</a> [SiF6]2−.

Thaumasite open-in-new

Thaumasite