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. After removing the supernatant (after the last wash), add ~25 mL of a 1 % Borax solution to the centrifuge tube (Figure 9, Step #8). Borax prevents the sample from flocculating. Too much Borax however will increase flocculation. 
2. Position the sample with the sonicating probe abut half-way into the sample material. Use the clamp to hold the tube in place (Figure 9, Step#9). The probe should not be touching the sides of the tube or the release of energy from the probe will melt the plastic tube. 
3. When satisfied with the positioning, flip the "ON" Switch (Figure 10, arrow A).  The settings are already set to our needs: Mode = Continuous, Amplitude = 65 (Figure 10). To run as is, press the "Test" button (Figure 10, arrow D), the screen will display 'rdy' when it is ready for use. If . Alternatively, if you select Mode = Time (min/sec), the probe will stop automatically after 1.5 minutes. If you need to change a setting, press the "Mode" button (Figure 10, arrow B) and then the "Set" button (Figure 10, arrow C). From these two panels, you have access to the power, time, and displayed units. For more information, reference the Manufacturer Manual located in the side pocket of the dismembrator case (Figure 10).
4. Press the "Start" button on the probe power supply (Figure 9, Step #10 and Figure 10, arrow E). If the material does not appear to be circulating throughout the whole tube or the tube is heating up, adjust the probe position and start the dismembration over. Allow the sample to circulate for at least 1 minute, then press the "Stop" button (Figure 10, arrow E). Remove the sample from the clamp. Wipe the probe with a Kimwipe, then clean with isopropyl alcohol and dry with a Kimwipe.
5. Leave the sample to settle for about 12 hours on the benchtop (Figure 9, Step #11).

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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