Photochemical degradation of a silicate in the beam of the electron microscope

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Abstract
In a recent paper (McConnell 1969) an account was presented of the electron optical study of incipient exsolution phenomena in a feldspar. During this investigation it was noted that the feldspar degraded in the electron beam to produce a porous and amorphous product. The present paper provides an account of this degradation reaction. The experimental observations indicate that the degradation reaction begins at the surface of the specimen and that its rate is approximately proportional to the electron flux through the sample. The rate is independent of the crystallographic orientation of the reaction interface and the thickness of the sample but is sensitive to the local chemical composition and is thought to proceed at a significantly higher rate in the sodium rich lamellae in the exsolution structure. In this context the reaction constitutes a high resolution photochemical etching process. Whereas the reaction rate is not in the main appreciably affected by the temperature of the specimen, it was found that in the temperature range where the sample was undergoing homogenization the degradation reaction was extremely rapid. The damage observed has been explained in terms of local ionization effects which produce a temporary charge imbalance and permit the loss of the alkali ions present by diffusion. Similar degradation phenomena have been observed in other alkali bearing aluminosilicates and in sodium glasses. The implications of reactions of this type in the interpretation of the results of electron probe microanalyses are discussed.

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