Crystal structures with a chabazite framework. III. Hydrated Ca-chabazite at +20 and –150°C

Abstract

cf. CA 58, 8478b. It was suggested previously that twinning of a low-symmetry structure with rhombohedral geometry gave the best explanation of the biaxial optical properties and distorted electron-d. peaks. The most likely cause of the low symmetry was ordering of the Si and Al atoms. The electron d. map for data collected at -150° is almost identical with that for 20°, thus ruling out the possibility that the distorted peaks are caused by statistical occupancy of several sites. The ordered pattern of Si and Al atoms, which is most likely to be chem. stable, has triclinic symmetry. Mimetic twinning would result in near superposition of 6 peaks for each atom when calcns. are based on rhombohedral symmetry. The 6 peaks for each Ca atom agree coincidentally. Other atoms are represented by single peaks, but many of these are either distorted or yield large artificial temp. factors at both temps.: this is interpreted as the result of near superposition of several at. positions. The intensities do not exactly obey the mirror plane in R̅3m, and this can be explained by imbalance between the twin units. Detailed analysis of peak shapes leads to a self-consistent set of coordinates in the triclinic cell P̅1. The directions of the distortions from rhombohedral geometry form a sensible pattern resulting in more reasonable interat. distances than those obtained from the averaged coordinates in R̅3m. The only features of the proposed structure that cause concern are 2 short Ca-H2O distances. Conclusive proof of the validity of the proposed structure can only come from x-ray analysis of an untwinned fragment. [on SciFinder(R)]