I have analysed clay particlesn with XRD and a peak associated with smectite was seen. I want to work out how much smectite could be in the clay by using XRF data I also obtained.
Reliable (powder) XRD needs separation, at least, of the clay fraction, that is less than 2 microns; as Dr. Towe told, you could take a look in the manual by Moore, D. M.; Reynolds, R. C., Jr. If your clay fraction is practically pure smectite, and you've already determined the type of smectite by XRD, then you may repeat XRF -bulk- analyses (or any other analytical method for Si, Al, Mg, Ca, K, Na, etc.) and consider the formula of smectite. Otherwise you maybe need careful point analyses (by electron microprobe with nano-sized beam, if possible)...
You can calculate smectite formula by transmission electronic microscopy coupled with energy dispersion X-ray (as you can see in my avatar), which allows to identify chemical composition from particle to particle with nano or micro size.