STEM-EDX spectroscopy and elemental mapping was carried out using the FEI Super-X detector and Velox software, under the same probe and acquisition conditions as for STEM-EELS. EDX spectra were used in-situ to distinguish kaolinite particles from impurity minerals on the basis of their chemical compositionfor example, muscovite in the low-Fe clay was distinguished by its potassium content, and gibbsite in the Fe-rich clay by its lack of silicon.