Chemistry of Smectitic and Illitic Phases in Interstratified Soil Smectite

The objective of this investigation was to determine the elemental composition and mineralogy of the smectitic and illitic phases in interstratified smectite-illite from a typical upper Midwestern agricultural soil. A sample of soil clay (<2-µm fraction) from the Ap horizon of a Webster (fine-loamy, mixed, mesic Typic Haplaquoll) pedon was fractionated by centrifugation to prepare various fine-clay (<0.020-, <0.026-, <0.036-, <0.045-, <0.060-, and <0.090-µm size fractions) samples. X-ray diffraction analyses indicated that interstratified smectite-illite was the only mineral present in significant quantities in the fine-clay samples. Elemental analyses of the fine-clay samples were performed by inductively coupled plasma-atomic emission spectroscopy using suspension nebulization. The elemental analyses revealed significant linear correlations for the K vs. other element relationships (P < 0.001 for K vs. Si, Al, Fe, Ca, Ti, Mn, and Zn, and P < 0.01 for K vs. Mg). A nonlinear parameter optimization technique, based on elemental mass balance, was used to estimate the elemental compositions and proportions of the smectitic (K-free) and illitic (K-bearing) phases in the fine-clay samples. The results for the elemental mass balance optimization were independently verified by water mass balance. The smectitic phase was a high-charge (0.482 per formula unit), Fe-rich montmorillonite (46.6% tetrahedral charge). The illitic phase consisted of dioctahedral, tetrahedrally charged (86.9% tetrahedral charge) elementary illite particles. Layer charge for the illitic phase was 0.473 per formula unit, which is substantially below the lower limit for classification of illite. The interstratified smectite-illite comprised at least 60% of the whole (<2-µm fraction) soil clay and the illitic phase comprised at least 36% of the interstratified smectite-illite.