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, an iron-rich clay mineral belonging to the smectite family, is characterized by a chemical makeup exceeding 30% Fe2O3 and less than 12% Al2O3.