NMR studies of heat-induced transitions in structure and cation binding environments of a strontium-saturated swelling mica

In this work, we combine Al-27, Si-29, F-19, and Na-23 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) to characterize the structure and interlayer cation environments in a strontium-saturated member of the swelling mica family before and after a heat-induced collapse of the interlayer space. The Al-27 and Si-29 MAS NMR demonstrate that the sample consists mainly of swelling mica, though the composition does not match the ideal structural formula. Aluminum NMR also shows that a portion of the aluminum shifts from a tetrahedral to an octahedral coordination environment upon heating. Changes in the Si-29 and F-19 NMR after heating are consistent with a structural rearrangement of the tetrahedral sheet to permit the binding of larger cations in the ditrigonal cavity. The Na-23 MAS NMR results indicate the presence of three unique sodium environments before and after heating. The heat-invariant resonance is consistent with the presence of sodium carbonate. The other two resonances are associated with interlayer sodium and reflect a migration of sodium to a dominantly anhydrous ditrigonal binding structure with heating. Quantitative elemental analysis and NMR data presented here suggest strontium is bound deep within the ditrigonal cavity of the collapsed micas.