Electron optical study of incipient exsolution and inversion phenomena in the system NaAlSi3O8-KAlSi3O8

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Abstract
In a recent paper (McConnell 1965) an account was presented of the mechanism of the polymorphic inversion in KAlSi3O8. This was based on the electron optical study of the microstructure of a natural material, adularia, which showed evidence of partial inversion. The present paper deals with the application of similar techniques to the study of the earliest stages in the processes of exsolution and inversion in a member of the binary series NaAlSi3O8-KAlSi3O8. Single crystal x-ray photographs of this material showed a monoclinic array of Bragg maxima with associated intense streaks of intensity normal to b. Electron diffraction photographs indicated that these strong streaks were aligned close to a and had maximum intensity in the a direction in reciprocal space. Associated intense electron diffraction contrast effects were observed and interpreted in terms of incipient exsolution into fine scale, lenticular regions enriched in sodium or potassium. An additional suite of streaks with much lower intensity was observed in the Laue zone Okl in the same material. In this case the streaks lay approximately in this zone and were orientated both parallel and perpendicular to b and an analysis of the intensity distribution of the streaks, and the electron diffraction contrast showed that two orthogonal transverse distortion waves were present. By analogy with similar circumstances in adularia (McConnell 1965) this distortion pattern was associated with incipient inversion to the triclinic state. Heating experiments, both in a laboratory furnace and on a heating stage in the electron microscope, showed that the contrast pattern due to incipient exsolution disappeared readily at a temperature somewhat above 640°C.

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