The nature of component layers of mixed-layer illite-smectite and their possible evolution in the course of illitization have been debated since the 1960s. The present study is a new attempt to solve these problems, using samples collected from diverse geological formations around the world. Twenty three purified illite-smectites from bentonites and hydrothermal rocks, covering the complete range of expandability, were analyzed chemically, including NH4+ determination, and their structural formulae were calculated. The exchangeable cations (EXCH) were plotted vs. the fixed cations (FIX) yielding the following experimental regression:

\[EXCH\ =\ {-}\ 0.43\ {\times}\ FIX\ +\ 0.41\ (R^{2}\ =\ 0.98)\]

FIX and EXCH depend on the charge of the illite (Qi) and smectite (Qs) interlayers, and the fractions of these interlayers in the bulk clay leading to:

\[EXCH\ =\ {-}\mathit{Q}_{s}/\mathit{Q}_{i}\ {\times}\ FIX\ +\ \mathit{Q}_{s}\]

Analysis of these relations and independent measurements of the total specific surface area (TSSA) indicate that the layer charges of both types do not change in the course of illitization. The smectite layer charge is equal to 0.41 and the illite layer charge is equal to 0.95 per O10(OH)2. End-member illite has a well defined composition that is close to intermediate between muscovite and phengite, with fixed cations content greater than that specified in the AIPEA classification of layer silicates:

\[FIX_{0.95}(Si_{3.25}Al_{0.75})(Al_{1.81}Fe_{0.01}Mg_{0.19})O_{10}(OH)_{2}\]

The established relationship allows the calculation of the mean number (N) of 2:1 layers in all fundamental particles and also the fraction of smectitic layers (fs) from FIX:

\[\mathit{N}\ =\ \mathit{Q}_{i}/(\mathit{Q}_{i}\ {-}\ FIX)\ \mathit{f}_{s}\ =\ (\mathit{Q}_{i}\ {-}\ FIX)/\mathit{Q}_{i}\]

N and fs can be used to calculate TSSA, and all three parameters can also be calculated from cation exchange capacity and from X-ray diffraction peak positions, utilizing the regressions established here.

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