The actual distribution of octahedral cations in 2:1 clay minerals: Results from clay synthesis

Abstract

Numerous 2:1 clay minerals were synthesized between 100 and 250°C. For each clay mineral, octahedral sheets were occupied by two different cations A and B, the atomic ratio R= A (A+B) varying from zero to one. Synthetic clays were studied by XRD and IR Spectroscopy with a view to determine the actual distribution of the two cations. Random distribution of octahedral cations is rarely observed, even for homovalent substitutions. In this case similarities in chemical properties (electronegativities for example) appear more important than similarities in ionic radii. For one (Al-Fe)-beidellite, with a R value of 0.5, I.R. data suggest an ordered distribution of the two cations. Segregation of octahedral cations is very common and leads either to clusters within octahedral sheets [kerolites and (Ni-Mg)-talcs, (Fe 3+Mg)-beidellites] or to an immiscibility of two different phases (Al-beidellite and Mg-saponite). So smectites with intermediate chemical compositions between di and trioctahedral end-members cannot be considered as homogeneous and random solid-solutions.