ANALYTICAL ELECTRON-MICROSCOPY IN CLAYS AND OTHER PHYLLOSILICATES - LOSS OF ELEMENTS FROM A 90-NM STATIONARY BEAM OF 300-KEV ELECTRONS

Diffusion of alkali and low-atomic-number elements during the microbea m analysis of some silicates by analytical electron microscopy (AEM) h as been known for some time. Our repeated analyses at 300 kV of kaolin ite, halloysite, smectite, biotite, muscovite and pyrophyllite, howeve r, showed differential loss (relative to Si) of not only alkali elemen ts (such as K, Na, Mg) and low-atomic-number elements (such as Al) but also higher-atomic-number elements (such as Fe, Ti). For AEM of these phyllosilicates, a Philips EM430/EDAX facility with a tungsten filame nt was used to provide a current of 0.3 nA in a stationary beam of nom inal diameter 90 nm. The loss of Al in kaolin minerals during analysis is particularly severe. Kaolin crystals can be damaged by the electro n irradiation over several seconds, making it the most sensitive clay to the electron beam; in general, relative phyllosilicate stabilities are kaolin < smectite < pyrophyllite < mica. A clear dependence of ele ment loss on crystallographic orientation has been observed for layer silicates in our study; a greater element loss occurred when the plane of the specimen foil was perpendicular to the basal planes of the phy llosilicate crystals than when the foil was parallel to the basal plan es. Lower beam current, larger beam diameter and thicker specimens all reduce the loss of elements. The initial stage of irradiation produce s highest rates of element loss and the rate of loss can be fitted by an exponential decay law. The analyses at low temperature of phyllosil icates showed that element loss remains serious in our analytical cond itions. Since the element loss appears to be instrument- and method-de pendent, one should use closely related, well-characterized phyllosili cates as compositional standards to calibrate any AEM instrument that is to be used to analyze unknown phyllosilicates, and the standards an d unknowns should be analyzed under identical conditions.