That leucite transforms from a tetragonal to a cubic polymorph with increasing temperature has been accepted for nearly a century, but the precise nature of this transition has provoked some disagreement. X-ray diffraction experiments suggest that leucite inverts directly from space group I41/a to la3d, however, two distinct absorption events appear in most calorimetric experiments, raising the possibility that leucite passes through an intermediate phase with space group I41/acd. In this paper, the evidence for both hypotheses is reviewed in detail.

Further, the transition between low and high leucite was examined in situ during heating experiments with a transmission electron microscope. Dark-field imaging with the 420 diffracted beam revealed pseudomerohedral and merohedral twin types in the low-tem-perature polymorph, and with increasing temperature the contrast among both sets of twins steadily diminished. The transition temperature Tc was deduced from the merging of split spots in selected area electron-diffraction patterns, indicating an equivalence in the lengths of a and c. Merohedral twins could not be discerned above Tc, whereas the pseudomerohedral twin boundaries definitely could be distinguished above the transition temperature.

We conclude that leucite can indeed occur as a distinct intermediate phase with space group I41/acd that is metrically but not symmetrically cubic within the error of our measurements. The failure of leucite to transform directly to a cubic structure may be related to Al/Si ordering. Further, the observation that leucite remains tetragonal above the nominal transition temperature explains the memory effect long associated with leucite.

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