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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 (Eggleton, 1977 Clay minerals,12:181-194; Keeling et al., 2000 Clays and Clay Minerals, 48:537-548). 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_.5(Si₇Al_.8Fe_.2)(Fe_3.5Al_.4Mg_.1)O₂₀(OH)₄ (Mountainville nontronite, Gates et al., 2002 Clays and Clay Minerals, 50:223-239). 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 (Bischoff, 1972, Clays and Clay Minerals, 20:217-223; Eggleton 1975 American Mineralogist, 60:1063-1068). Some evidence suggests that microorganisms may play an important role in their formation (Kohler et al., 1994 Clays and Clay Minerals, 42:680-701).