APPLICATION OF BSE TO STUDY OF SEDIMENT GRAINS

There are two main ways in which backscattered electron microscopy (BSE) images can be useful in the study of loose sediment grains. In both cases it is first necessary to embed a representative subsample of grains in a resin block to allow preparation of a polished surface (not necessarily a polished thin section). If it is necessary to examine the textural relationships of the grains in situ, without disturbance to the primary sediment fabric, resin impregnation can be carried out in the field. Otherwise, impregnation is normally undertaken in the laboratory after homogenization and splitting to provide a representative subsample.

As discussed in Chapter 2, the atomic number contrast evident in BSE images allows many of the common minerals in modern sediments and soils to be differentiated. In situations where there is a relatively small number of mineral species present in a sample, and where each species has a significantly different backscattering coefficient, it may be possible to determine quantitatively the relative abundance of different minerals. Figure 3.1 shows a heavy mineral beach placer deposit from Queensland, Australia, which consists of a mixture of quartz grains and heavy minerals, chiefly ilmenite and zircon. The abundance of heavy minerals in a se-ries of such micrographs can be determined by grain counting, either manually or automatically by using dedicated computer software. By adjustment of the contrast mechanism, zircon and ilmenite can be differentiated and counted separately. The abundance of different minerals can be estimated by setting gray-level thresholds to define a “window” specific to each mineral detected (e.g., Ball and McCartney, 1981; Pye, 1984a).