Preceding studies assigned a cubic symmetry (Ia3d) to analcime, even if possible lower symmetries are suggested from contradictory evidence: e.g., the occurrence of optical anomalies and the presence of forbidden reflections in X-ray photographs.

In an attempt to check the true symmetry of analcime, the crystal structures of seven optically anisotropic and apparently untwinned crystals from various localities were refined (final R from 0.02 to 0.07 for the observed reflections). They were tetragonal (I41/acd, with cell edges: a > c in four samples and a < c in one sample) and orthorhombic (Ibca). These different symmetries follow from the different ordering of Al in each of three tetrahedra and from the related different occupancy of the nearest Na site. Neither tetrahedra occupied only by Al (maximum Al fraction 0.5), nor complete occupancies of Na sites (maximum 0.84) were observed.

Simple relationships were obtained between Al fractions in tetrahedra and (1) occupancies of the nearest Na sites, (2) lattice parameters, and (3) intensities of triplets of X-ray reflections.

A hypothesis is given regarding the possible derivation of all the crystal structures of analcime from the superposition of three different orientations of one basic tetragonal structure, with an Al fraction of 0.5 in two equivalent tetrahedra and no Al in the third.

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