The effect of the octahedral cations on the dimensions of the palygorskite cell

High-resolution synchrotron X-ray diffraction recorded on a collection of palygorskites with different chemical compositions (obtained by analytical electron microscopy) permits unambiguous correlation of the crystallographic parameters a (or a sin beta if a monoclinic phase is considered) with the nature of the octahedral sheet, i.e. with both the number of octahedral positions that are occupied and the type of octahedral cation. No significant changes in the lattice parameters b and c are observed. The unit cell modification consists essentially of an expansion in a as the number of cations with larger ionic radii (Mg2+ and Fe3+) predominates over smaller cations (Al3+). A linear dependency of a (or a sin beta) on the chemical composition of the octahedral sheet was obtained that can be used for classifying palygorskite into compositional groups, using only conventional diffraction data, without the need for chemical analyses.