Local Structure of the Interlayer Ion in Synthetic Fluorine Mica

Abstract
Na-form tetrasilicic fluorine micas, i.e., sodium taeniolite (NaT, NaMg₂LiSi₄O₁₀F₂.nH₂O) and sodium hectorite (NaH, Na_1/3Mg_8/3Li_1/3Si₄O₁₀F₂.nH₂O), exchanges their Na⁺, the interlayer cation, for K⁺, Rb⁺, Ca²⁺, Ni²⁺, and other cations in solutions. NaT shows remarkably high selectivity for K⁺, Rb⁺, and Cs⁺ and NaH does for Rb⁺ and Cs⁺. XAFS of the exchanged ions in NaT and NaH were analyzed and powder X-ray diffractions were measured to investigate the layer stacking. Characteristics of ion-exchange selectivity of these micas were revealed by the local structure of the interlayer ion. Rb K-edge EXAFS of all samples show slightly longer Rb-O_1st distance than K-O_1st in taeniolite (its original anhydrous K-form). It indicates that Rb⁺ for the micas is exchanged as an anhydrous state. X-Ray diffraction patterns showed hydrated interlayer-ion phases at low-exchanged NaT and NaH. The hydrated phases decreased with increasing the exchange ratio and then less-hydrous Rb-phase increased. Although Cs⁺-exchanged NaT show only an anhydrous state in XAFS, Sr²⁺ and Ba²⁺ in NaT was hydrous as if in aqueous solution.