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Nontronite
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% Fe2O3 and less than ~12% Al2O3 (ignited basis). Nontronite generally does not exist in economic deposits like montmorillonite, although it is not an uncommon clay mineral.[4][5] Like montmorillonite, nontronite can have variable amounts of adsorbed water associated with the interlayer surfaces and the exchange cations. Additional recommended knowledgeA typical structural formula for nontronite is Ca.5(Si7Al.8Fe.2)(Fe3.5Al.4Mg.1)O20(OH)4.[6] The dioctahedral sheet of nontronite is composed mainly of trivalent iron (Fe3+) cations, although some substitution by trivalent aluminium (Al3+) and divalent magnesium (Mg2+) does occur. The tetrahedral sheet is composed mainly of silicon (Si4+), but can have substantial (about 1 in 8) substitiution of either Fe3+ or Al3+, 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 (Ca2+) or magnesium (Mg2+). Nontronite forms from the weathering of biotite and basalts, precipitation of iron and silicon rich hydrothermal fluids and in deep sea hydrothermal vents.[7][8] Some evidence suggests that microorganisms may play an important role in their formation.[9] References
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Nontronite". A list of authors is available in Wikipedia. |