The Coordination of Aluminum Ions in the Palygorskite Structure

NMR spectra of PF1-1 Floridan palygorskite strongly suggest that Al ³⁺ occurs only in octahedral coordination. X-ray microanalysis of the palygorskite fibers indicate a chemical composition defined by the atomic ratios: Mg/Si = 0.34, Al/Si = 0.27, and Fe/Si = 0.04. Considering the NMR and CEC data in this report along with the previously published results of IR and Mössbauer spectroscopic studies, the following structural formula is proposed for PF1-1 palygorskite: $${\rm{(M{g_{2.12}}A{l_{1.68}}F{e^{3 + }}_{0.24}{\square _{0.96}})S{i_8}{O_{20}}{(OH)_2}{(O{H_2})_4}}}$$ where □ represents the vacant Ml octahedral sites in the structure, and Al ³⁺ and Fe ³⁺ are all exclusively assigned to the octahedral sites.