Application of analytical transmission electron microscopy to identifying intercrystal variations in the composition of clay minerals

Abstract

Two examples of the use of analytical transmission electron microscopy (AEM) in solving mineralogical problems are reported. In the first instance, a beidellite-type smectite was identified using X-ray diffraction (XRD) in the clay fraction of a lateritic pallid zone from south-western Australia. The beidellite mineral constitutes <5% of the kaolin-dominated clay fraction of the sample and, thus, it is not possible to derive the chemical formula using bulk chemical analysis. The AEM analysis of beidellite indicates that this mineral is similar in chemical composition to an ideal beidellite, but is anomalous in that it contains large amounts of non-exchangeable K (K₂O = 2.2%). The second example is the determination of Fe in soil kaolinites using AEM. Analyses of the bulk clay fractions of soil kaolinites indicate the presence of ≈2.4% Fe₂O₃. It is uncertain to what extent this Fe is present as discrete impurities (such as iron oxides) and whether it is a constituent of kaolinite. Furthermore, the bulk analyses gave no indication of whether there was a range in values of Fe content between individual kaolinite crystals. AEM analysis of twenty single crystals for each of seven soil-kaolinite samples confirmed the presence of 1.96–2.83% Fe₂O₃ in kaolinite and that there was a considerable variation (s values of 0.66–1.21% Fe₂O₃) in iron content between kaolinite crystals in each sample.