The fundamental problem which you've got is that minerals don't have a constant composition. When you get substitution of one element for another, then quite probably both the peak positions and their intensities will change. In the case of plagioclase, you've got substitution between Ca and Na, and simultaneously between Al and Si, to maintain the charge balance. If you've got access to the book "Mineralogy" by Ivan Kostov, there's a rather helpful table on p. 394 showing the variation in d-spacing and intensity for 7 or 8 strong diffraction lines across the composition range. Different peaks move in different directions, and the positions can vary by 0.08 angstroms. In principle you can use these shifts to give you a clue as to the composition, but I doubt if you'd find it easy! The best approach would be to use chemical analysis data - have you got access to XRF? - to get an idea of where in the compositional range your sample lies.

Have you been able to separate the individual minerals? Doing so will make the analysis of the diffraction pattern a LOT simpler. Techniques to use, if you've got access to them, will include density separation (heavy liquids; and/or shaker-table with flowing water - miner's pan if you haven't got the automated version!); magnetic minerals can be removed by magnetic separation (if you've not met this, it's much like the shaker-table but uses a magnetic field to effect the separation rather than water flow)

This takes me back to when I was a student many years ago - my undergraduate project involved these sort of techniques to separate out titanite, and my doctoral work looked at species related to your other post, acid mine drainage.

Good luck!

David

Thank you so much David Beveridge - I do have XRF analysis data (attached) but I am still not sure how to use that XRF to compare to XRD data to get a better sense of which "height of main peak " fits which mineral? Maybe you can please suggest a better approach?

Thank you so much Doriana Vinci - yes, I agree, there are many clays (as secondary minerals) in my samples. I will implement your advice.