Positions of cations and molecules in zeolites with the mordenite-type framework I dehydrated Ca-exchanged ptilolite

Abstract

The crystal structure of dehydrated Ca-exchanged ptilolite has 4 sites for Ca: one each in twisted and near-circular 8-rings, one on the wall of the main channel, and one in a boat-shaped pocket in a zig-zag channel. Hindrance but not blocking of molecules could occur. The TO distances correlate inversely with TOT angles, consistent with observations and molecular-orbital calculations for feldspar. Tetrahedral mean TO distances adjusted for this effect indicate strong but not complete Si, A |l disorder. The Ca occupancy is consistent with simple electrostatic and geometrical arguments.