²⁹Si Magic Angle Spinning Nuclear Magnetic Resonance, Fourier-Transform Infrared, and Monte Carlo Study of Synthetic...

The parallel ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR) and Fourier-transform infrared study of synthetic micas made it possible to compare structural features of the tetrasilicic magnesium mica K(Mg_2.5□_0.5) Si₄O₁₀(OH)₂ (TMM) and their K(Mg₃)(Si_3.5Mg_0.5)O₁₀(OH)₂ (TMMA) and K(Mg₃)(Si_3.5Be_0.5)O₁₀(OH)₂ (TMMB) derivatives. In the TMM mica, SiO₄ tetrahedra are elongated in the plane ab and shortened along the c* direction with respect to those of the phlogopite (Phl) K(Mg₃)(Si₃Al)O₁₀(OH)₂. The substitution of Si⁴⁺ by R²⁺ (Mg²⁺ or Be²⁺) produces, besides the ²⁹Si MAS NMR signal of Si (3Si) at -91.2 ppm, new components at -84.4 or -87.5 ppm that correspond to Si (2Si1Mg) or Si(2Si1Be) environments. Tetrahedral cation distributions in TMM/TMMA, TMM/TMMB solid solutions are investigated with respect to the TMM/Phl series by means of NMR and Monte Carlo simulations, concluding that divalent Mg²⁺ and Be²⁺ are further dispersed than trivalent Al³⁺ cations in tetrahedral sheets of micas. In three analyzed series, cation distributions display features between those of the homogeneous dispersion of charges of phlogopites and the maximum dispersion of charges of TMM derivatives. In three series, the location of charge deficits that compensate K⁺ cations changes from octahedral in TMM to tetrahedral sheets in phlogopite and TMMA and TMMB derivatives.