Thaumasite is a calcium

silicate mineral Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

, containing Si atoms in unusual

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

configuration, with chemical formula Ca₃ Si(O H)₆( C O₃)( SO₄)·12 H₂O, also sometimes more simply written as CaSiO₃·CaCO₃·CaSO₄·15H₂O. It occurs as colorless to white prismatic

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A '' regular hexagon'' has ...

crystals, typically as acicular radiating groups. It also occurs as fibrous masses. Its

Mohs hardness The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by t ...

is 3.5 and it has a

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...

of 1.88 to 1.90. Optically it is uniaxial negative with indices of refraction of nω = 1.507 and nε = 1.468. Thaumasite-251460

It occurs as a

hydrothermal alteration Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...

mineral in sulfide ore deposits and geothermal alteration of

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

and

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

. It occurs with

zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H+. These p ...

apophyllite The name apophyllite refers to a specific group of phyllosilicates, a class of minerals. Originally, the group name referred to a specific mineral, but was redefined in 1978 to stand for a class of minerals of similar chemical makeup that compris ...

analcime Analcime (; ) or analcite is a white, gray, or colorless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is Na Al Si2 O6· H2O. Minor amounts of potassium and calci ...

, calcite,

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywal ...

and

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

. Thaumasite can also be formed in man-made

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...

structures at the detriment of

calcium silicate hydrate Calcium silicate hydrate (or C-S-H) is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials (e.g. concrete). Preparation When water is added to cement, each of the compounds ...

s (C-S-H, with dashes denoting the non- stoichiometry of this hydrated cement phase acting as the "glue" in hardened cement paste) during

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...

alteration, especially when sulfate attack develops. The reaction consuming the silicates of the "cement glue" can lead to harmful decohesion and softening (more rarely to expansion and cracking) of

. Unlike conventional sulfate attack, in which the calcium hydroxide (

portlandite Portlandite is a hydroxide-bearing mineral typically included in the oxide mineral class. It is the naturally occurring form of calcium hydroxide (Ca(OH)2) and the calcium analogue of brucite (Mg(OH)2). Occurrence Portlandite occurs in a variety ...

) and calcium aluminate hydrates react with

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...

s to form

and

ettringite Ettringite is a hydrous calcium aluminium sulfate mineral with formula: . It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part of ...

(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

Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th ...

may be affected. It was first described in 1878 in Sweden and named from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

, "thaumazein", ''to be surprised,'' in reference to its unusual composition with

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

, sulfate and hydroxysilicate anions. The silicate structure of thaumasite is unusual due to the presence of non-tetrahedral silicon in its crystal lattice. Indeed, an atypic

configuration is observed for Si present in thaumasite in the form of hexahydroxysilicate: i(OH)₆sup>2−, a species exhibiting a geometry similar to that of the

hexafluorosilicate Hexafluorosilicic acid is an inorganic compound with the chemical formula . Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless. Hexafluo ...

iF₆sup>2−.

References

External links

Thaumasite form of sulfate attack (TSA)
Calcium minerals Carbonate minerals Cement Concrete Hexagonal minerals Minerals in space group 173 Hydrates Luminescent minerals Silicate minerals Sulfate minerals

See also

References

Further reading

External links