Twinning in tetragonal leucite |
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| Authors: | David C Palmer Andrew Putnis Ekhard K H Salje |
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| Institution: | 1. Department of Earth Sciences, University of Cambridge, Downing Street, CB2 3EQ, Cambridge, England
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| Abstract: | The change from cubic to tetragonal symmetry in natural leucite, KAlSi2O6, involves two types of twinning, which appear sequentially with decreasing temperature: (1) lamellar, associated with the point group symmetry reduction m3m to 4/mmm; (2) merohedric, associated with the change 4/mmm to 4/m. Twin orientations have been deduced from X-ray precession photos and TEM diffraction patterns and images. These are confirmed by theory, using the concept of spontaneous strain and the symmetry relations between adjacent domains. Lamellar twins have boundaries parallel to {101} of the cubic phase, and are cross-cut by the merohedric twins, which have irregular boundaries with x and y in adjacent domains interchanged. On an electron microscopic scale, the distortion arising from twin intersections is revealed by a curvature of one twin wall adjacent to another, and by needle-shaped domains. The macroscopic orientation of the lamellar twins is related to the lattice parameters and hence the magnitude of the spontaneous strain. This in turn leads to the characterisation of the order parameter for the m3m to 4/mmm transformation. |
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