Crystal-size control and characterization of Na-4-mica prepared from kaolinite

A simple and economical method for the synthesis of high-charge-density Na-4-mica (ideal chemical composition of Na₄Mg₆Al₄Si₄O₂₀ F₄·χH₂O) from kaolinite was investigated. Synthesis conditions were varied to control crystal size. This novel swelling Na-4-mica was synthesized from a mixture of kaolinite, magnesium nitrate (or ultrafine MgO) and NaF at 850°C, and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), particle size analysis by laser diffraction and ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. Decreasing the mass of NaF flux during the crystallization of Na-4-mica reduced the crystallite size from 2 to 0.2 μm. As the crystallite size decreased, the average particle size of the mica decreased from 5 to 2 μm. ²⁷Al MAS-NMR spectra showed that all or nearly all Al is in tetrahedral coordination whereas ²⁹Si MAS-NMR spectra showed that the nearest neighbour environment of Si is mainly in Si(3Al), as expected based on the Si:Al ratio.