A compilation of 221 space-group corrections from a false low symmetry (FS) to a higher true symmetry 0108-7681/92/040356-14506.00 (TS) shows that higher symmetry is often overlooked in only a few space-group types. An incorrect lattice (false crystal class) is found most often for rhombohedral space-group types, and there especi-

The crystal structure of the Li-exchanged form of natrolite, Li 1.6Na04AJ,Si301O . 2H20, was refined based on 568 F obs' R = 0.071. For comparison the structure of a natrolite, NazAJ,Si30lO· 2H10, was also refined (1278 F obs' R = 0.040). The diffraction data collected for K-natrolite were of insufficient quality, therefore a computer simulation of the K-form was based on the experimentally determined cell edges and space group. The mean rotation angles ljI of the chains (composed of 4 = 1 secondary building units) relative to the a and b cell axes are observed to be 250 (Li-compound), 240 (Na-compound) and 180 (K-compound). A detailed comparison shows that only the transformation to K-natrolite conforms approximately to the currently widely accepted model of rotating chains. Surprisingly the Li-form instead achieves the observed shortened cell constants a and b by a distortion of the chain itself, the rotation is only a secondary effect. Both cation exchanged forms display also shortened c-axes, which is an indication that the chains of 4 = I units are tilted and twisted within themselves. It is most likely that the angle of the hinge at the oxygen atom 02 is in Na-natrolite already at the lower attainable limit beyond wh ich it would become energetically unfavorable, and thus it cannot be reduced any furt her when the Na-natrolite is transformed by ion-exchange into a Li-natrolite.

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