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 has very few economic deposits like montmorillonite. Like montmorillonite, nontronite can have variable amounts of adsorbed water associated with the interlayer surfaces and the exchangeable cations. Ignoring the isomorphic substitution of Si by Al, with exchangeable cations compensating for the resulting negative electrical charges and their hydration water in the interlayers, the elemental composition of nontronite could be idealised as simply FeIII2Si4O10(OH)2. A typical structural formula for nontronite is Ca0.3(FeIII3.5Al0.4Mg0.1)(Si7Al0.8Fe0.2)O20(OH)4·nH2O. 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) substitution of either Fe3+ or Al3+, or combinations of these two cations.