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. Like montmorillonite, nontronite can have variable amounts of adsorbed water associated with the interlayer surfaces and the exchange cations.