Powder X-ray diffraction (XRD) was used to identify the mineral phases present in the as- Thermogravimetry was used to confirm that full dehydroxylation of kaolinite had taken place during calcination. A Netzsch STA 449 F5 was used, with a heating range between 30 and 1000°C, and a heating rate of 10°C/min, in an N2 gas atmosphere with an N2 gas flow rate of 60 mL/min. A sample mass of 20 mg was used in alumina crucibles. Fitting parameters are provided in Table in the Supplementary Information. The uncertainty in quantification within each spectrum was estimated to be equivalent to the % signal to noise ratio in each respective measured spectrum. The standard Thermofisher double-tilt sample holder and regular vacuum conditions were used. In TEM mode, the magnification was limited to <185 kX maximum with an accelerating voltage of 300 kV and a probe current of 200 pA. Figure shows resolution of unaltered basal planes of kaolinite in an image acquired under these conditions of a stack of kaolinite in the as-received low-Fe clay. The kaolinite (001) lattice spacing was measured as ~0.71 nm, considered to match well with a value for fully hydrated kaolinite of 0.74 nm obtained by X-ray diffraction data . No significant plane warping or breakdown was observed, and this level of beam intensity was therefore assumed to be on or below the threshold for significant damage to kaolinite in the microscope. The dwell time of the electron probe was limited to ~1 s, in order to ensure minimal damage per dwell point. Spectra were collected for 30 s continuous scanning and then background subtracted (power law) in 'Gatan Microscopy Suite' software (v 3.0.1). In order to reduce spectral noise, a 40 point moving average was applied to all spectra before displaying the final results and the 'typical' spectrum of at least three representative spectra presented.