Showing papers by "Ray L. Frost published in 1985"

Ordering of aluminum in tetrahedral sites in mixed-layer 2:1 phyllosilicates by solid-state high-resolution NMR

Abstract: Al and Si MAS NMR results are presented for the mixed-layer 2:1 phyllosilicate rectorite. Separate Si spectra, obtained by partially relaxed inversion-recovery experiments, for the two types of tetrahedral sheets present are consistent with one sheet being smectite-like, i.e. low aluminum occupancy of tetrahedral sites, and the other sheet being mica-like, i.e. high aluminum occupancy of tetrahedral sites. Comparison of the relative areas of the Si(0-3Al) resonances from the mica-like sheets with those predicted by computer modeling indicates a high degree of ordering of substituted aluminum. The observed areas indicate that minimization of both Al-O-Al (Loewenstein's rule) and Al-O-Si-O-Al (Dempsey's rule) interactions is important. For the tetrahedral sheets Si/Al ratios derived from Al MAS spectra indicate a much higher aluminum occupancy than determined by XRF or Si NMR.

Solid-state silicon-29 spin-lattice relaxation in several 2:1 phyllosilicate minerals

Abstract: Si NMR spin-lattice relaxation times have been measured for the two 2:1 layered phyllosilicates, sepiolite and rectorite. In sepiolite, it is shown that water contained within the structural channel provides a Si-H dipolar contribution to Si spin-lattice relaxation. In rectorite, hydrated interlayer paramagnetic cations provide very efficient relaxation either via spin diffusion or, more likely, via direct electron-nuclear relaxation facilitated by relatively high cation mobility. Of the two types of tetrahedral sheet in rectorite, the one further removed from the paramagnetic ions exhibits a relaxation time approximately 15 times longer than that for the closer sheet. Removal of the paramagnetic cations by exchange causes a significant increase in spin-lattice relaxation times for both types of silicate sheets. However, a smaller relaxation time difference still exists through a Si-H dipolar contribution to one type of sheet from the interlayer water. This contribution disappears when the interlayer water is removed by heating. The presence of considerable contributions from these mechanisms to Si spin-lattice relaxations in solid silicates will clearly be beneficial where the use of Si-H cross-polarization is either impossible or undesirable and where long relaxation times, typical of rigid solids, would otherwise be expected.