NMR of albite–microcline series

Abstract

The chemical bonding in silicates has been studied experimentally and theoretically¹. Spinning at 54.72° yields sharp peaks in NMR patterns for ²⁹Si and ²⁷Al (refs 2, 3). Qualitative interpretation of the isotropic chemical shift is useful, but the interrelated effects of framework geometry, cation substitution and hydrogen bonding must be quantified for zeolites. The isotropic chemical shift for ²⁹Si in silica polymorphs is related to the mean secant of adjacent Si–O…Si angles⁴, and the complex NMR patterns of tridymite and uncalcined fluoride silicalite have been interpreted quantitatively (see also refs 5, 6). In some zeolites⁷, the average Si–O–T (tetrahedral) angle is correlated with the isotropic chemical shift for ²⁹Si. The proposed relationships^8,9 between mean Si–O distance and the isotropic chemical shift for ²⁹Si are poorly obeyed by silica polymorphs⁴, and the cation-oxygen bond strength provides a better correlation than Si–O for a wide range of silicates¹⁰. We show here that the mean secant Si–O–T provides a better guide than Si–O and bond strength for Na,K-feldspars in the series between low albite and low microcline.