Powder X-ray diffraction must be done with fine-grained samples. Clays are already fine-grained, but other things may have to be crushed as fine as is practical for best results. Note that some soft minerals (e.g., carbonates, gypsum) may undergo structural damage during grinding, which can result in distorted patterns.
These are the typical sample holders for routine XRD analysis on the old Philips machine. The well mounts generally give more reproducible results for crushed samples, but require more sample and greater care. The glass slide mounts are easy to make and generally give good results for clays. They require less sample, but the results tend to be poor for crushed samples, because surface roughness is difficult to control.
Well mounts: Prepare a fine-grained sample by sieving, or by grinding it in a mortar and pestle, or both. The sample is put into the middle of the well and pressed flat with a glass slide. Several pressings are usually necessary, in between cleaning off powder from around the well. It’s important that the top of the sample be smooth and coplanar with the top of the aluminum sample holder.
Glass slide smear mounts: Make a liquid slurry and spread it smoothly onto the slide surface. Pop any bubbles. Blowing on the surface, or gently shaking the slide from side-to-side can help evenly distribute the sample. Clays usually stick by themselves, but other samples may need a binder. A dilute solution of transparent dish detergent in water (1 drop to 10 ml of DI water) works pretty well. It can be the liquid that the slurry is made with. Alternatively, after the sample is dry on the slide you can wet it with a dilute solution of Duco Cement® in acetone (1 drop to 10 ml of acetone). Clays tend to shrink and curl up on smooth glass, so frost the slides first using a 400 grit silicon carbide slurry on a glass plate. The tape (e.g., Scotch® tape) shown in the illustration is needed to make the sample surface approximately coplanar with the goniometer rotation axis.