Stabilization of alkali earth metal cations in Na-4-mica

Na-4-mica, a highly charged swelling mica, has suffered from insufficient understanding of some fundamental features such as partial layer collapse and rearrangement of interlayered cations and layers during exchange reaction. This study focuses on elucidating the changes in chemical environments of divalent cations and in layered structure during the exchange reaction with divalent cations. Cation exchange reaction with a series of alkali earth cations led to significant changes in both XRD pattern and IR spectra. The extent of cation exchange reaction depended highly and systematically on cation size and hydration energy. Exchange reaction with Mg²⁺ and Ca²⁺ led to highly hydrated phases whereas that with Sr²⁺ to anhydrous phase and that with Ba²⁺ to remarkable degradation of layered structure. The hydrated divalent cations could be quickly stabilized in interlayer space in spite of localized incompatibility of both lattice and charge through formation of interstratified phases having various ratios of Na⁺/M²⁺ and Na⁺/H₃O⁺. Therefore, divalent cations could be stably accommodated either in hydrated or anhydrous state within the cavities, preferably around crystal edges to obstruct their further uptake.