MAS AND DOR NMR-STUDY OF AL-SI ORDER IN THE ANALCIME-POLLUCITE SERIES

Al-Si order in the analcime - pollucite series (ideally NaAlSi2O6.H2O -CsAlSi2O6) was studied using magic angle spinning (MAS) and double ro tation (DOR) nuclear magnetic resonance (NMR). The Al-27, Si-29, Na-23 and Cs-133 NMR spectra of natural and synthetic samples show signs of increasing disorder from analcime to pollucite, but the synthetic pha ses may contain up to about 50 mol.% pollucite without a significant i ncrease in disorder compared to end-member analcime. Cubic pollucite i s the most disordered; however, complete disorder is not observed, eve n in rapidly formed. synthetic end-member pollucite. In the cubic fram ework, cations are not randomly distributed among tetrahedrally coordi nated sites, but follow an Al-O-Al avoidance rule and probably also th e extended Loewenstein rule. Analcime and cesian analcime show an incr eased degree of order with symmetry less than cubic, but remain incomp letely ordered. The influence of Si/Al ratio on degree of order was de monstrated; with increasing Si/Al, intensity shifts in the Si-29 spect ra from the Si(3Al) and Si(2Al) peaks to the Si(1A1) peak.