Influence of Aqueous Si and Fe Speciation on Tetrahedral Fe(III) Substitutions in Nontronites: a Clay Synthesis Approach
Résumé
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 [4]Fe(III) content (x) ([Si4–x Fe(III) x ]Fe(III)2O10(OH)2Na x ) was synthesized at 150ºC 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 d 0633 value of 1.562 Å was measured by X-ray diffraction (XRD) for the highest charged nontronite (x = 1.54). This high d 0633 value has not been reported in the literature for a dioctahedral smectite until now. The [4]Fe(III) content (x) of the synthetic nontronites, estimated using Fourier-transform infrared spectroscopy (FTIR) through the wavenumber of the main stretching nSi–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. H3SiO– 4(aq) and H2SiO– 4(aq)), 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.