What is "nontronite"

Nontronite \Non"tro*nite\, n. [So called because found in the arrondissement of Nontron, France.] (Min.) A greenish yellow or green mineral, consisting chiefly of the hydrous silicate of iron.

n. (context mineralogy English) An iron-rich clay mineral of the montmorillonite group

Nontronite is the iron(III) rich member of the smectite group of clay minerals. Nontronites typically have a chemical composition consisting of more than ~30% Fe O and less than ~12% AlO (ignited basis). Nontronite generally does not exist in economic deposits like montmorillonite, although it is not an uncommon clay mineral. Like montmorillonite, nontronite can have variable amounts of adsorbed water associated with the interlayer surfaces and the exchange cations.

A typical structural formula for nontronite is Ca( SiAlFe)(FeAl Mg)O(O H). The dioctahedral sheet of nontronite is composed mainly of trivalent iron (Fe) cations, although some substitution by trivalent aluminium (Al) and divalent magnesium (Mg) does occur. The tetrahedral sheet is composed mainly of silicon (Si), but can have substantial (about 1 in 8) substitiution of either Fe or Al, or combinations of these two cations. Thus, nontronite typically is characterised by having most (usually greater than 60%) of the layer charge located in the tetrahedral sheet. The layer charge is typically balanced by divalent calcium (Ca) or magnesium (Mg).

Nontronite forms from the weathering of biotite and basalts, precipitation of iron and silicon rich hydrothermal fluids and in deep sea hydrothermal vents. Some evidence suggests that microorganisms may play an important role in their formation. Microorganisms are also involved in reduction of structural iron in nontronite when soils undergo anoxia, and the reduced form of the clay appears to be highly reactive towards certain pollutants, perhaps contributing to the destruction of these compounds in the environment.