Most dioctahedral 2:1 swelling clays in natural systems contain ferric iron, Fe(III), which can be located in both the tetrahedral and the octahedral sheets. The distribution of Fe(III) between octahedral and tetrahedral sites in nontronite depends on the Fe and Si speciation during nontronite synthesis. The role played by the chemical properties of solutions in the Fe(III) distribution between structural sites was studied through nontronite syntheses. A chemical series of Fe(III)-nontronites with variable tetrahedral Fe(III) content (x) ([Si4−xFe(III)x]Fe(III)2O10(OH)2Nax) was synthesized at 150°C across a range of initial aqueous pH values between 11 and 14. The permanent layer charge, due to Fe(III)-for-Si(IV) tetrahedral substitutions only, ranged from 0.43 to as high as 1.54 per half-unit cell. A value of 1.562 Å was measured by X-ray diffraction (XRD) for the highest charged nontronite (x = 1.54). This high value has not been reported in the literature for a dioctahedral smectite until now. The Fe(III) content (x) of the synthetic nontronites, estimated using Fourier-transform infrared spectroscopy (FTIR) through the wavenumber of the main stretching νSi–O band, was correlated with synthesis pH and its influence on calculated aqueous Si speciation. The increase in synthesis pH induced the increase in anionic aqueous Si species ratios (i.e. and ), and favored the incorporation of Fe(III) in tetrahedral sites of synthesized nontronites. During nontronite formation in natural systems, the level of tetrahedral Fe(III)-for-Si(IV) substitutions may, therefore, be partly linked to the aqueous Si speciation and thus strongly dependent on the pH of the crystallization fluids.