Electron optical study of effects associated with partial inversion in a silicate phase

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Abstract
Electron optical technique has been used to study the mechanism of the polymorphic transformation in KAlSi3O8. This transition involves a reduction in symmetry from monoclinic (C2/m) to triclinic (CI) with falling temperature and is associated with ordering of the aluminium–silicon distribution on tetrahedrally coordinated structural sites. The transition involves in addition only minor changes in the lattice parameters. The transformation is kinetically sluggish, a condition which results in the existence of stranded or metastable structural states characterized by partial inversion phenomena. Such effects are observed in adularia which shows Bragg diffraction maxima in monoclinic array associated with intense streaks on single crystal diffraction photographs. Study of the microstructure of adularia using the electron microscope indicates that the monoclinic lattice is homogeneously perturbed by two orthogonal transverse distortion wave systems. Nodal regions in this distortion pattern correspond to the two alternative triclinic orientations demanded by the symmetry reduction from C2/m to CI. The structure observed is regarded as a transitional state from which coarse inversion twinning derives by selective growth of certain nuclei at the expense of others. The latter process constitutes the rate controlling step in the transformation. The electron diffraction contrast effects associated with the distortion waves have been explained in terms of current kinematic theory of electron diffraction contrast.

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