Clay Separations focus on separating the clay size fraction, <2 µm, from the rest of the material. In order to get only that size fraction we prepare the sample in a different way compared to bulk analyses. Do not freeze dry samples waiting for clay separations.

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1. Add 25 mL of 1% Borax solution to the clay plug 
2. Dismembrate the sample (machine is auto set on time), to remove the >2 µm clay fraction
3. Centrifuge for 4 mins at 750 rpm, decant the supernatant liquid into a separate centrifuge tube (you should end up with a ~full centrifuge tube of suspended clay)
4. Repeat steps 1–3 on the remaining >2 µm fraction
5. Centrifuge the <2 µm fraction for 15 mins at 1500 rpm to remove the Borax solution 
6. Decant and add 25 mL of nanopure water
7. Centrifuge for 60 mins at 3000 rpm, the liquid is decanted before loading onto a zero background silica disk.

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1. Take the <2 µm clay fraction, this should be rinsed and free of any treatments.
2. In centrifuge tube, add ~20 mL of 1N HCl.
3. Vortex the sample to completely dislodge all material before pouring sample into a 100 mL beaker, add stir bar once sample is in place.
4. Set hotplate to 300°C (boiling point of HCl is ~101ºC, but in order to maintain a continuous boil the hotplate needs to be much hotter)
5. Set the stir RPM below 100, this is only to keep the material suspended within the HCl
6. Start time once the samples have come to a complete boil, leave for 2 hours — add more HCl as necessary (DO NOT let the sample go dry and burn)
7. Once the sample has boiled for 2 hours, turn off the hot plate and allow sample to return to room temperature before pouring beaker contents back into a clean centrifuge tube. Rinse beaker with DI water to collect all material
8. Centrifuge down for 15 mins at 1500 rpm, decant acid and rinse with 25 mL nanopure (DI) water. Repeat the washing cycle 3 times.
9. After samples has been washed 3 times and is free of HCl, centrifuge for 60 mins at 3000 rpm

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References

Jackson, M.L., 1956. Soil Chemical- Analysis Advanced Course by Hsueh-Wen Yeh, Hawaii Institute of Geophysics, 1980.
Moore, D.M., and Reynolds, R.C., Jr., 1989. X-ray Diffraction and the Identification and Analysis of Clay Minerals: New York (Oxford University Press).

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XRD Sample Preparation Clay Separations - September 27, 2022