Field emission scanning electron microscopy a high-resolution technique for the study of clay minerals in sediments

High-resolution field emission scanning electron microscopy (FESEM) is achieved through use of a field emission gun (FEG) in place of the more conventional tungsten hairpin or LaB6 filaments. A resolution of 0.5 nm has been achieved under ideal conditions and 5 nm is regularly achieved at 30 kV with conducting materials (Fig. 1). However, with insulating materials such as rocks this figure is closer to 50 nm. The amount of clay data which can be acquired using back-scatter or conventional secondary electron imaging techniques is limited by the resolution of such instruments. For clay mineral studies the optimal resolution is 0.5 p.m for back-scattered electron microscopy (BSEM) or 2.5 nm for conventional secondary electron microscopy (CSEM) compared with 0.5 nm for FESEM (Table 1). Good quality photomicrographs can be obtained at a magnification of x 2,000 for BSEM, x 5,000 for CSEM and x 30,000 for FESEM. In favourable circumstances (i.e. observations of electrically conducting materials rather than rock samples), photomicrographs of acceptable quality can be obtained at x 15,000 magnification for CSEM and • 50,000 for FESEM. In view of the highresolution imaging offered by FESM observations, it is surprising that FESEM has not been more widely adopted for the petrographic analysis of fine grained rocks. M E T H O D O L O G Y A field emission gun produces an electron beam with an extremely high current density, obtained by applying an intense electric field to a tungsten single crystal with a needle-shaped tip. Unlike the conventional thermal emission gun this requires ultra high vacuum (10 -1~ Torr). The FEG is often referred to as the high brightness gun because the emission source for electrons is extremely small and because the number of electrons emitted per unit area is much greater than with other types of electron guns. This, together with the much 9 1997 The Mineralogical Society 198 J. M. Huggett and H. F. Shaw